"Who must file" is not the same as "who must pay." If, for example, you have assembled enough itemized deductions and other things that you owe zero taxes--I don't know if that's literally still possible--the IRS will want the details of how you did it.
And, frankly, if I'd filed regularly year after year, I'd probably go ahead and file even if I weren't required to, because not filing might raise some electronic eyebrows.
Straight from the horse's mouth:Do You Have To File?
It says "use chart A," and if you scroll down a bit there it is.Do I have to File a Tax Return?
points out "some instances when you may want to file a tax return even though you are not required to do so," such as refunds of withholding tax, credits, etc.
My personal experience is that they do pretty decent "customer service" and that it is NOT a waste of time to telephone them at 1-800-829-1040 if you have questions.
Annual income twenty pounds, annual expenditure nineteen nineteen and six, result happiness; Annual income twenty pounds, annual expenditure twenty pounds ought and six, result misery.