There are many reasons why multiple filesystems are advantageous. There are also some disadvantages. We will begin by listing the advantages in decreasing order of importance:

Size limitation: The size of the portion of a filesystem that resides on a separate partition is limited by the size of the partition. Thus, areas of the filesystem which may allow unbounded creation of files should be placed on a separate physical filesystem so that their growth does not compromise the rest of the system. Examples of such areas are:

• /tmp - this world-writable directory is a tempting location for the storage of videos, ISO images and the like. However, users of space on /tmp often neglect to delete data they no longer need. The directory can often grow unbounded.
• /home (or wherever home directories are). - this directory is writable by users and, as such, can grow unbounded. It is also wise to separate out /home to protect user data from system data. Then if the system is corrupted, the user data may remain intact (see below)
  
• /var - this directory contains log files, rpm files (for installation), and spool files. In particular, log files can be problematic. An early technique for compromising a system, especially those that allowed remote logging, was to write to its log file repeatedly. If /var is part of the root filesystem, system function may be compromised by a runaway log file.
  Note: /var also contains a public temporary directory /var/tmp. Although this is most often used by programs, it is subject to the same problems as /tmp. If /var is not a separate partition, at least /var/tmp should be. Since /var/tmp is world-writable, this can allow unprivileged users to compromise system logging by filling up the /var filesystem before an attack that would normally be logged! (On many systems, these issues are avoided in one of two ways: /tmp is cross-mounted at /var/tmp using the bind option (making them the same filesystem) OR /var/tmp is a symbolic link to /tmp).
  

• If a hardware failure occurs in a disk component, only part of the Unix filesystem is affected. This factor is less important today since many systems implement multiple physical filesystems as multiple partitions on the same physical disk drive. Since failure in a disk drive today often results in loss of the entire drive, every filesystem stored on that drive will be lost.
• The most vulnerable time for a filesystem is during write operations. If you compare two parts of the filesystem: the part that houses user files and the part that houses the system files, user files are updated much more frequently. Since most system files are static, the separation of system data on a separate physical filesystem offers some protection from any data corruption that occurs in other, more frequently updated, areas. This is true as well of the most crucial part of the system - the kernel. Often the kernel is stored on a separate system, /boot, to protect it. (On current systems /boot is separate as well to keep it as a physical filesystem if the rest of the system is under the control of a logical volume manager. The current version of grub cannot boot from a LVM filesystem.)
  

Efficiency: Frequent updating of data causes data fragmentation and disk randomization. Isolation of system data on a separate physical filesystem ensures that when the system needs space, the space is less fragmented and is not intermingled with user data.

Ease of backup: Some backup programs function on a filesystem by filesystem basis. User data, of course, needs to be backed up much more frequently than the more static system data.

Flexibility: Any non-root filesystem can easily be moved to another system and attached to their filesystem if necessary. In addition, different physical filesystems can employ different filesystem parameters, such as blocksize, based on their use, or even be formatted in a different filesystem type. Last, some filesystem parameters, such as the use of access-control lists must be applied on an entire physical filesystem. Such parameters may be appropriate for user files but not system files, or vice-versa.
One unusual, but useful, advantage to multiple filesystems is creating a data partition to share between several bootable systems. On my system, for example, /doc is a partition that may be mounted on two different systems at boot. You could also share a partition for downloaded special programs. Typical locations for this are at /opt or /usr/local

Besides complexity, the single most significant disadvantage of using multiple filesystems is sub-optimal space allocation. It is easy for a filesystem to be created with either more, or, more importantly, less, space than is eventually needed. Although there is a tool to resize a physical filesystem, it cannot resize it past the boundary of the parititon. This makes increasing the size of a filesystem usually difficult, unless there is free space following it. The only solution to this problem is to split the filesystem into two physical filesystems or to create a new larger partition to place it on. Both solutions involve temporarily denying access to the data while all or part of it is copied to the new location and the reconfiguration is complete.

As we will see later, logical volume managers provide all the important benefits of multiple physical filesystems while providing the ability to grow a filesystem without recreating it. 

Most systems that are divided into multiple physical filesystems have at least /tmp and /var separated from /. Another common division is /home, which contains user data. On linux systems, /boot is also commonly on its own partition. /boot contains the most basic system startup components including the boot loader configuration files and the kernel.

In addition to 'real' partitions, modern Unix and linux systems should have a swap partition. This effectively multiplies the amount of 'virtual memory' available on the system. ('virtual memory' used like this is a PC-inspired misnomer) 

Some filesystem divisions can be very inconvenient. The root filesystem is understood to contain everything necessary for a functioning system. Under no circumstances should /bin, /etc, /dev, /root, /lib or /sbin be on a different partition than the root. In addition, while it is traditional to place /usr on a separate partition, the dynamic libraries in /usr/lib and many files in /usr/share are now an integral part of the system, so this may no longer be a good idea. If the intention is to separate local data from system data, a separate partition may be mounted at /usr/local instead to house additional source and binary installations.

Find out what filesystems your Unix or linux system uses. The current filesystems can be displayed using the df command. If a filesystem device starts with the characters lv or vg or vol, the filesystem is probably a logical volume under the control of a logical volume manager. How many separate filesystems comprise your system's Unix filesystem?