That’s more of a general Linux question. In part traditional, yes, but some reasons are still valid today, depending on the setup and requirements.
If you use separate partitions (or rather: file systems), then filling one partition (Logs in /var/log, user files in /home/*, …) does not affect the others. You could even use different file system types. And have different mount options. Depending on your backup solution, separate file systems could also be required if you want to implement different backup or snapshot policies. Or put one file system on a disk on fast (but expensive) storage, other parts on slower but cheaper storage. @tosch already mentioned easier resizing. Perhaps implement quotas per file system. …