First, storing var and home under different partitions still remains common practice, even on a server.

Those folders contain information that are truly important in case of failure of the root folder.

Most folders are expendable. Backup can only get you until the last saved restore point, up to 24hs late if auto backup once a day. That's a full day of work gone.

var and home keep you safe and up to date. A custom install and you're ready to roll or migrate.

Second, a swap file can easily be corrupted if the parent partition is damaged or unreachable for some reason. The advantage is that it can be resized far more easily than a partition.