The point of tipping is that it's _supposed to_ go straight to the servers, while normal charges go the owners who then share them with their employees as they see fit.

Assuming this is true, and tips are not actually taken by the owners, there are two interpretations of this:

1) customer pays more than what their lunch or dinner actually costs, to intentionally signal that there was something special about it (good service,...), or

2) customer pays more than what their lunch or dinner actually costs, because they know employees are underpaid and they want to compensate for that. It's essentially a form of charity.

While I have nothing against charity, charity that is "institutionalized" (big word, I know) by the employees (a tip jar) and very strongly encouraged does seem like a nice way for employers to justify a lower pay. And even if that is not the actual reason why tips exist, an employer who knows most of its customers tip can use this to pay less its employees.