It's true that if the only device you own is a Mac, then iCloud offers less than it would to the mobile-device-plus-Mac owner.
However, there's still plenty of good stuff there for anyone. Here's a partial list:
1. Ad-free, IMAP email
2. An Apple ID for the iTunes/Mac App/iBook/iTunes U/etc stores
3. Find My iPhone/iPad/iPod touch/Mac (yes, Mac!)
4. (Sorta coming soon) iCloud Drive, where you can store up to 5GB of stuff free, and more for a very modest fee.
5. (also coming soon) iCloud Photo Library, if you wish, can automatically back up your photos.
6. And of course when (not if!) you gain an iOS device, all these services and more spill over onto them with automatic syncing.
7. Use of the web-based iWork suite for free -- Pages, Numbers, and Keynote.
8. Convenient web access to Notes, Reminders, Calendar, Contacts and Mail when you are away from your own computer
There's probably a thing or two I've forgotten, but all that for $0 ain't a bad deal.