The main logic behind it is that the ground you run at in competition is not always the same conditions you run at home.
If you are running on hard, fast ground, you may need a bit with a little more rate to control your horse better.
If the ground is muddy and slippery, you may need a light bit that gives your horse more freedom to move as needed.
If it's a large pattern, again, you may need a bit with more rate as you approach each turn.
If a small pattern, you want a free-running bit to encourage your horse to stretch out.
So that's kind of the concept. Of course, this is not switching to a harsher bit for the sole purpose of covering up training problems and "controlling" the horse
with it. Rather, you use different bits to help set your horse up for the best success with the conditions you are running under. Especially in rodeo where ground conditions are never as ideal as nicely groomed NBHA's.
And certainly there is no rule that says you have to switch. But I've read a lot of books by the pro's and they all have a trailer-full of bits they are constantly switching to and from based on the running conditions. And I'm guessing they know a thing or two.