[neoden]

> you can run out of space at one partition, but have lot of free space at another

that's exactly the point — you can run out of space in your /home but that does not affect, for example, /var. or vice versa, log explosion in /var is contained within its own partition and does not clog the entire filesystem.

[codedokode]

Logs should had a limit, and dealing with partitions is inconvenient for home user. How do you easily change allocation of space between partition?

Also, there are quotas for this purpose.

[PunchyHamster]

very important for /var/log, pretty easy to have log spamming app fill the drive, and you don't want logs filling get your database into out of disk space state