> Breathing is deeper and calmer and more powerful.

Diaphragm breathing is also part of the training for classical flute players to breathe. It is how babies breathe naturally.

and then?

I did some quick googling and it seems like there's some preliminary research that indicates it might help you reduce stress[0]. But I wonder how this stacks up against going for a jog or riding a bike, which have a well established record of reducing stress, and have other benefits.

[0]: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5455070/

Complementary and synergistic. Why pick breath training or vigorous exercise when you can just do both?

I think it leaves you physically stronger and mentally calmer. YMMV but it works for me.