They do, it’s grouping those operations to say that they have the same precedence
They don't. It's irrelevant that they have the same priority. MD and DM are both correct, and AS and SA are both correct. 2+3-1=4 is correct, -1+3+2=4 is correct.
Without them it implies you always do addition before subtraction, for example
And there's absolutely nothing wrong with doing that, for example. You still always get the correct answer 🙄