It's a legacy thing from running UNIX machines with small'ish hard drives. A rogue process' log file could fill the root volume and prevent logins to the system

It's a legacy thing from running UNIX machines with small'ish hard drives. A rogue process' log file could fill the root volume and prevent logins to the system. That's why /var/log or /var was on its own mountpoint/partition. It was much more of a UNIX sysadmin thing than a LINUX sysadmin thing, as EXT2 reserved space for root to do things.