They slightly increase your top speed, up to a maximum of 10 coins.
@Mike, did you see what happened to me yesterday in one of our online races? :
The player using Morton knocked me out of my original powerslide trajectory and I went straight into the Goomba!
Funny thing was I kept accelerating trying to drive through it, and it took a few seconds to realise it wasn't working.
That's the first (and hopefully last) time I ever need to use reverse in MK8.