Umbrellas are standard carry item at big shoots.........offer to hold it for other shooters and they'll return the favor.
It helps with really bad lighting conditions where you have sun in your eyes trying to see targets in shadows, glare on your peep etc. A small travel umbrella will work
We had one person who brough one in our group at the 2nd Leg of the IBO Triple Crown 2 years ago and it got quite a bit of use.
You won't have a "Caddy" at these shoots as it's just the guys you are shooting with but generally an agreement can be made of "I'll scratch your back if you scratch mine"
It's a big help under certain circumstances.

Really heavy wind is another time you'll see them. In that case you should plan ahaead and bring the full sized golf umbrella. You can use it likea walking stick while you stroll around or rig something up on your shooting chair.

Trust me, at the big shoots you will kill yourself for bringing a hip quiver and not a shooting chair. I've sat for 1/2hr at some targets during a back up. They are great for toting all your crap, food, water, arrows, umbrella, tools etc.....and taking a load of your legs 15 targets in on a really hot day is worth it's weight in gold.