Thoughts on Filesystem Granularity
The reason for this is that over the years I've seen situations where some daemon went bonkers, logging voluminous amounts of verbiage to something in /var/log, thus filling up /var (or worse, if /var wasn't a separate partition of its own, filling up the root filesystem); or where some user copied (or tried to copy) a massive file to /var/tmp, with the same result; and so on.
If /var, /var/log, and /var/tmp are kept in separate disk partitions, it helps quarantine the damage when some daemon (or some user) gobbles up all the available free space in one of them. If a user fills up /var/tmp, at least it doesn't keep syslogd from continuing to log information in /var/log; if some daemon generates so much syslog verbiage that /var/log fills up, at least other things which need to create (or append to) files elsewhere in /var are still able to do so; and so on.
It's worth noting that an advantage of having /usr/local in a separate partition is that if it's ever necessary to blow away the OS, scrub the partitions, and reinstall from scratch, it's easy to do so while still keeping /usr/local untouched. And in the case of re-imaging a cluster node with xCAT or SystemImager, the OS can be installed without blowing away the previous contents of /var/log, which is often useful to keep around.