To expand a bit...

> Having one big partition may be OK for a home system, but for a system
> that may be on the internet (or multiuser) there are many good reasons
> for having multiple partitions. The main ones being security and system
> integrity. With one big partition you run the risk of filling up the
> partition and making your system unusable. This can easily be done by
> filling up /tmp, /var, etc. Making these separate partitions reduce the
> risk of bringing your system down if one of these fill up.

Keeping /home on a separate partition allows you to redo your whole system
(e.g., remove one distribution and install another) without losing user
data. Same applies to things like all your non-pkg installations in
/usr/local/.

Then there's backup.  It's a easier to design and implement backup
schedules for different parts of the filesystem that have different
requirements (e.g., /home changes much more frequently than /usr) if
they're on different partitions. 

I don't know what else. I'm in Texas at the moment, and so I can't think. 

Andy