[quantummagic]

You're asking how a tip can influence a server, if they don't know it's coming. But in America they do know it's coming, at least, there is a cultural norm of tipping being expected. So it makes sense that a server would do what it takes, to make sure that it happens.

[kelnos]

So in other words, you'd still get good service without tipping at all in places like the US. Granted, it might be awkward when it comes time to leave, and if they'd recognize you, you may not want to go back.

[quantummagic]

Yes, there are antisocial people who disregard social norms. But there are enough people who play along to have a major impact on how service providers behave.

[MH15]

That's what the money is for!

[quantummagic]

Huh? The OP asked how a tip could influence the service, if it came after the service was delivered. It's pretty easy to understand, when a tip is the cultural norm.