If I understand it correctly there is a lot of overhead on normal internet communication to make sure that packets are sent and received correctly.
One of the things Gamemax does is drop some of the overhead.
Now if you were to do that on your machine then all your downloads/streaming etc. would become unreliable (parts of your download never reaches you but your machine does not care).
It improves your latency because every command you give is not sent & confirmed, it is only sent, but it makes your internet unreliable.
With something like a MMORPG it is not the end of the world if a packet is missed here or there.
I saw a while back there is a WoW latency hack that you can do to your PC to achieve the same result (if you already use an unshaped account) but like I said, it corrupts your downloads.
Using a 3rd party that only drops the overhead on WoW traffic makes a lot of sense.
P.S. I use Gamemax on Vodacom 3G and when the 3G connection is not sh|t (as it usually is) i get 300 - 450ms pings. Without it I get 400-600ms.