The bellhop/valet is just standing around waiting for some chance to perform a free service in hopes that maybe he can get a tip. He’s likely near minimum wage for his time and tips are an expected to actually make it a worthwhile activity. That said, I don’t carry cash often enough and regularly don’t tip these services any more. I try to avoid using them if I can’t tip. Sometimes that’s not an option, like a restaurant that only has valet and no nearby parking alternatives. In those cases, I let them work it out with the restaurant.
To be clear, I was listing examples of traditionally tipped jobs. That wasn't comprehensive.