Back in the days, what often happened was /usr was mounted over NFS or a second harddisk like you mentioned (space and maintainability being possible reasons).

Back in the days, what often happened was /usr was mounted over NFS or a second harddisk like you mentioned (space and maintainability being possible reasons). Because NFS locking was an issue, you'd use lockable files (such as /var and /tmp) on different partitions.