Partitions can be used to protect critical components of a system from less critical ones. For example, it is common to use one partition of critical OS specific components, and another partition for user data. This way, if an application program “runs away” and create large files, it will only fill up the user data partition. The OS partition remains untouched, and will still be able to function and log messages that can later be used to diagnose the problem.
Some file systems support encryption, this is another means to protect (the privacy) of a system. Due to performance consideration, it is not necessary nor desirable to encrypt all partitions. In our example, the OS partition may remain un-encrypted, while the user data partition is encrypted.