It's traditionally used in yoga, and can be used to direct energy where you want it. Even though it is said that it opens up more space for prana to collect (because you're directing your attention to your belly area (where the sub-navel storage area is), I think it's personal preference.
I was told by a practitioner that because it's different than 'normal' breathing, you have to put your mind to it, and it sets up an intention and creates an association with whatever it is that you're doing, like any successful ritual.