I hope this is a joke...
The advantage of wavedashing, you can vary the speed and distance, often faster then running, almost no start-up frames, No cool-down frames, you can use any ground attack except the dash attack from a wavedash.
It doesn't win matches on it's own, but all those advantages improve your overall game if you know how to use them properly and can wavedash well.
Erm, ok. If you waveshine correctly, guess what, you'll get your next waveshine in during your opponent's recovery frame. Recovery frames=no counter, dodge, attack etc.
Which allows you to rack up some awesome damage or, (more often) set your opponent up for a shinespike.
It only seems like a useless trick because you're not doing it properly, you need near-perfect precision otherwise your opponent can counter, and a cpu will punish you for even an instant of vulnerability. They are ungodly precise but very easy to trick.