I already have Genesis and it's great, i cant stop playing it. But i also wonder what MOTBW offers beside graphical enhancements. Is there a reason to buy that game?

By default, MotBW's single player campaign is the same as that of Genesis but unfolds much more quickly, so if you are finding the campaign length to be an issue then it may be worth trying MotBW.

There are a few additional bits of content such as special shard properties and desert provinces, although those could probably also be modded into Genesis.