Some reasons other than Dual Boot:
- Separation of User Data from System/Application data.
For example, I can back-up a Disk E with all my user data. This answer given in more detail in some other answers to the question
- Prevent overload of one disk from killing whole system (it's a UNIX thing)
In the old days, your mail server or your log files might fill up your all your disk space, causing many headaches for the sys-admin, including the ever humorous "108% full" disk drive.
- Data protection, if some partitions are set "read only"
This was a common Linux, and perhaps other, security thing. You put your kernel and some other files on a "read only" partition to prevent malicious users from changing those files. When it was time for an upgrade, the sys-admin had some hoops to jump through to get the partitions writeable.
To tell the truth, for single user machines, I find partitioning to be too much of a headache. I don't even dual-boot anymore, I use some kind of Virtual Machine.
For servers, the partitioning scheme depends on what kind of services the machine will be offering.