Most of the time it's to get them to actually do their job; shitty pay, bad or nonexistent oversight, corruption (or just "bribes") in higher levels too, conditionally applied rules, etc...
That creates a de-facto auction for their time and service, you (and what you're adding on top) vs. everyone else (and what they are adding on top). There's an unofficial "price list" depending on how quick and thorough you want things to be done.
"Tipping culture" easily slides into "bribing culture" when it becomes an expectation, regardless of what the Supreme Court says about it.
