Stuff you buy from Amazon can be automatically added to Amazon’s Cloud and accessed from whatever device you want. What’s the difference?
The difference is iTunes Match will scan your ripped music, identify matches with iTunes Store offerings, and make those songs immediately (ok, give it a few minutes) available on all your devices - rather than having to upload all XXGB of stuff over a prolonged period.
I could have my 46GB collection synced with iCloud by tomorrow for $25/yr. Amazon will cost a lot more and take a lot longer.
For free, you get every song you've ever bought from iTunes. For $25/yr., you can have every song you ripped from disk, bought from Amazon or Wal-Mart, torrented, recorded yourself, or pirated downloadable from anywhere. That's downloadable, not streamable. If you have lower-quality songs that are in the iTunes catalog, you can download them in 256K AAC.
I can now have any song in my 40GB iTunes library on my 16GB iPhone in seconds from anywhere. It's freaking huge; aside from the convenience, as Mashable put it, Apple has found the first way to monetize pirated content.