Silent echoes don't need the BoolArg. At least not when I've used it.
Although, if you're setting an opposite like for freeze (freeze has 3 parameters, calling player, target, boolean). The boolean is used to check if the Opposite command (unfreeze) or the normal command (freeze) is used. When the opposite is used, it's set to "_, _, true". Two first underscores are just to basically be the same as normal (calling player and target player), but the last one is to make the third parameter to "true", making it unfreeze.
It might be confusing (I'm tired), but basically: It's only needed for booleans, if you have something like "should_unfreeze", and then checks inside the command where it's like "if not should_unfreeze" (if should_unfreeze is false, basically, normal command).
Let's hope what I've understood is correct. Don't kill me if I'm wrong.
(My 200th post! Wooh!)