Most people forget that the federal minimum wage for tipped employees is lower than regular employees. They think that the tip is something extra that doesn't have to be given, but that's not true. Tipped employees have a right to that tip. Anyone who's worked as a server at a restaurant will know how terrible it is to not be given tip.
Tip is also meant to be an incentive for the employee to do their job better because the better they work, the more tip they will receive. But some people are stingy and no matter how well someone works, they don't like to leave much tip. If only they put themselves in the shoes of that tipped employee!