with my very limited understanding of the tactic :
it's a trade off,
play with a back 4 and perhaps the CMs can play higher up the pitch
VS playing with a back 3, allows the 2 wingbacks to push higher up the pitch (unless you are Mick McCarthy).
Ive seen it said that 3-4-3 for example makes more "natural passing triangles" for playing out from the back than 4-4-2 for example- but I'm not entirely sold on that concept