PocketMac Backup is the first Pocket PC-to-Mac backup tool. It backs up the entire Pocket PC, not just PIM data, but also installed programs, documents, registry & much more.
While it's true that PocketMac (or ActiveSync) will sync most of your contact, calendar and files, there's still a lot of information, applications, system files, registry data, drivers, etc. on your Pocket PC that just won't get synched.
Just syncing your contacts & calendar regularly is not enough to ensure a full system backup. You can also selectively backup or restore only those parts of the system that you want.