Long: The string catching your arm guard is usually the result of placing the guard in the wrong place, though shape and thickness of the guard can also cause it.
They're probably not needed, except to keep clothes out of the way, for shooting most recurves that have high brace heights, say 7-9 inches. If you shoot a straight gripped long bow with a brace height of 6 1/2 inches or less, you'll soon discover the need for an arm guard.
Of course, some folks shoot with a substantially bent bow arm, which also keeps the forearm out of the way of the string.
I rather agree with Old Goat. Course, I'm an old goat myself.