This is called the Pudding Raise and is bid on a hand with 4-card support for partner, an even distribution and 13 - 15 points. Here, without this bid, you would have to respond on a 3-card minor (you'd never respond on a 3-card major!) grotty 4-card minor, and then bid 4♥ next time, which really does't describe your hand very well. Alternately, you would jump to four hearts and partner won't know if you have a good raise to four or a weakish very distributional hand. But, with this bid in your armoury, a jump to four hearts is then only made on a weakish very distributional hand.
If declarer also has even distribution, then he may pass the 3NT bid, especially in pairs, for the extra 10 points.