The better your guesses about future betting, the better your gauge on implied odds, and the better your understanding is of what the right decisions are. It's just another thing you can do better than your opponents to gain an edge.

+EV is short for positive expected value. Expected value is the average profit you expect to get from something. So if something is +EV, that means on average you're going to get a positive profit from it (as opposed to a negative profit aka losing money).