Before I began with aikido training I did 8 years yoga and our teacher alwayst told us to breath deep by expanding the abdomen, and also we had to breath out in the double of time we did to breath in. There are a lot of benefits you can read
here. But most of all it makes you relax yourself and it is easy to verify, when you do a lot of training and your breathing is too fast try it and your breathing will become slow faster.