Keep in mind that the main function of your abs is to stabilize your body. What the ab wheel promises is that exercising with an ab wheel makes all your core muscles work together and to contract as a group. 21 healthy adults were asked to do a series of core exercises while connected to a device that measures the strength of a muscle contraction.
The ab roller with which you perform the wheel rollout exercise is known for the intense challenge it places on the abs. These are the primary muscles worked by the ab roller. While rolling out primarily targets the lats and deltoids rolling in targets the core specifically the rectus abdominis.
In essence its a very good exercise for the upper body.