It helps for leverage.
It helps for breathing.

It hurts for navigating tight trails.

I put some 74mm bars on a recent build and they are certainly wider than any of my old bikes. Not too comfortable yet as i need to now figure out a stem length and angle that effectively places me in the same position as before. I get that shorter stem is for a wider bar, but there is still some tweaking that needs to be done because of the wide bar. lame.