WAVs are pretty outrageous when it comes to size because they have no compression. MP3s are compressed and lose fidelity.
WAVs will sound the best of anything, and allow you to convert to your desired format - FLAC, AAC, whatever - at the best possible quality. if you're not planning to do that, or to do something professional with them, there's not much point in purchasing WAVs.
I'm pretty sure the WAVs are only offered to shut up anyone who moaned about the 160 kbps for In Rainbows. lol @ charging for the extra bandwidth.