Because if they don’t match something is wrong, you’ve ‘lost’ some money (well usually lost, I suppose it’s possible to gain money and not be able to account for where it can from but I’d guess it’s very rare). Yes the irs want to know that you’re paying the right amount of taxes but more importantly you as, a business owner, want to know that a) your business is making money and it’s worth while continuing and b) no one is stealing from you.