Well, they are more of an alternative to Nurbs surfaces as oppsed to polygons. Here are a few text book reasons.

#1. They can exist on arbitrary topology, such as polygons surfaces, bypassing the difficulity of creating a form in four-sided patches ( Nurbs ).

#2. They are smooth and continuous, like Nurbs surfaces. They do not have the problem of creating a faceted look, as polygons do.

#3. They allow a hierarchy of as many as 13 levels of detail, which allows isolated areas of highly detailed modeling and allows binding at base levels.

Hope that makes sence to you.