Joerg Sonnenberger added to the partion discussion:

The alternative for /tmp is to have lots of swap and MFS for /tmp. This is often faster and avoids the lots of old crap in /tmp problem.
In that case you should make /var/tmp its own partition. In general /tmp and /var/tmp as world writable locations should be on partitions
on there own. Making /usr/obj a filesystem of its own has the advantage
of faster cleaning — just unmount, newfs and remount it :)

He also noted that having specific partitions for things like news spools (/news/) and mail stores (/var/spool/) is that it allows the blocksize to be set much smaller, which decreases wasted space when dealing with lots and lots of small files.