It regulates your heart rate and relaxes the entire body. It also helps you focus on something calming.

This best answer to breathing meditation:

They say that breath is related to the spirit - in most languages there is a connection. But in many ways it is not actually that important. Focussing on the breath is one technique that became famous because it was the technique that buddha taught to beginners, but actually there are a myriad of other meditation techniques. One of my favourites is focussing on your distractions - whatever distracts you from the meditation - meditate on that!