What is the benefit to upgrading coaches and is it worth it?

When you add coaches, add the full 20 at a time instead of adding 5 or 10 here or there. It doesn't work like most expect. You are firing you're current coaching staff and hiring a new staff, so you will pay the sign on bonus each time you make any changes to the coaching staff. As mrcasado said, add fitness coaches first at 20. The next time you make a change have 20 fitness and 20 defense coaches. After that, I recommend waiting until you can afford 20 in fitness, defense, attacking, and tactical. If you prefer, you can skip the second step to save that sign on bonus.