It's a cultural thing.
It's a "compassionate" thing.
In the eyes of some it's a "character" thing.
Call it "tangible kindness" or "calculable appreciation".
Some might say that the way someone tips can be used as a way to gauge the quality of a person. It's sometimes expected, sometimes surprising, now and then I see a new place for it - or supposed new place.
Everyone's got their own opinions here.
I think that if a guy is bold enough to plop himself down and start playing music, with his guitar case open, he's "open for business". And if you like his busking, you should feel free to share some coin with him.
Walk by without dropping something in the case...nobody should fault you. Hang around and listen, without showing your appreciation...that's another story.
Some rely on it to make ends meet.
Some see it as gravy.
As gondoliers, tips play an active part in our world.
Anyone who's been around the block a few times will agree, that tipping is something you approach delicately:
Refer to it in passing, as if it's just "part of a story you're telling", and you can effectively remind your passengers of the practice. Speak it into existence too brazenly, and you run the risk of annoying your clients.
This annoyance could be reflected in your tip, or worse, you could get no tip (along with a complaining phone call to the office on Monday).
On a recent cruise vacation in Mexico, I noticed a prime example of an over-the-top annoying approach:
I'm sure this guy relies in-part on gratuity, but his signage was so blatant that I didn't see one person step up and put something in one of his many cups.