While Yoshi doesn’t always get a big role in every Mario game, I do remember his role here very well. 

I forgot Shadow Mario (Bowser Jr.) had captured his egg and you had to rescue the egg to get Yoshi on your team. 

After that you can ride Yoshi just like Super Mario World but all in 3D. 

Which gave Mario the ability to complete some missions impossible for even his FLUDD. Instead of water, Yoshi can shoot out some strange stomach juice. No questions asked there.