On the one hand, that's not how this works (it's automatic, not mandatory) -- even when it's added, you can still tip less than the amount on the check (or not tip at all). I believe the check is required to inform you of this by law, at least some places.
Originally Posted by Nanuk
On the other hand, one reason it's done is because those who favor it know that plenty of people think it really is mandatory and will pay it without looking closely or questioning it.
I always liked the checks that give you the bill and then the tip amount for various percentages underneath as a note (usually 10%, 15% and 20% although I hear that these days, 20% is the new 15%).
"If ye love wealth greater than liberty, the tranquility of servitude greater than the animating contest for freedom, go home from us in peace. We seek not your counsel, nor your arms. Crouch down and lick the hand that feeds you; May your chains set lightly upon you, and may posterity forget that ye were our countrymen." -- Samuel Adams