1.) Send directions to a mobile device. There are many times where I search for directions or map out an alternate route on my desktop. The only way I can access those directions is to email it to myself then open the email on the mobile device. Why not just store the data and let the user retrieve it on their mobile device?
2.) Better accuracy of property in rural/suburban areas. It's odd to say this but Bing Maps is actually much more accurate in rural areas.
3.) More fine grained control over street view placement.
4.) Offline maps! Not everywhere has good cell coverage.