As others have said, it's nice to just pick up your phone off the pad, use it, and put it back, without having to unplug & plug it back in.

But personally, there's one much bigger reason wireless is so much better than USB: Qi doesn't wear out your USB jack. After plugging & unplugging my phone at least once a day for a year or two (and occasionally bumping it while plugged in), the USB jack eventually became an extremely finnicky, barely-usable pain in the ass.