YALMIP and MPT both rely on each other. YALMIP uses MPT to solve the parametric programs, and MPT uses YALMIP to do some of the modelling.
If you have a very standard MPC problem, MPT has ready-made functions where you simply give A,B,N etc and it computes a solution, and has ready-made functions for simulations and analysis. YALMIP is used to extend the model, or create models from scratch when you have less standard problems