Because of following reasons you need MPLS:-
1. In legacy IP network, if you have to offerVPN services, In that case the customer routes need to be propagated in the core.Really a cumbersome job, just for customer routes we need to do the sizing of core routers.In fact thecustomer routes need not be required in the core routers. So MPLS help to make core customer routes free.
2. MPLS works as a transport network with access agnostic.
3. Customers can run any protocols and SP intervention is not required.
4. All customer can use the same ip adress range.MPLS help to distinguish thedifferent customer traffic.
There are many more...If you have some specific requirement, please share.