Every public bus I've been on in the US has one, there is usually a button somewhere by the doors also though

They're still on some of the older classes of tram in Melbourne. They're convenient because they run the length of the tram, so are less likely to be obstructed by other passengers during peak travel, and you can signal your stop without having to move very far in the carriage while the tram is moving (and they often do brake suddenly, so being able to signal a stop without letting go of a support for too long is good).

They're above head-height for most people, roughly at the level of the hand hold straps, so aren't wholly accessible, so there are retrofitted(?) buttons along the poles throughout the carriages.

I like having both. They do sometimes get ripped out by people who grab them to support themselves (not sure if there's also people who deliberately pull them out)

The new trams have absolutely TRASH button layout. You basically have to either rely that someone else is getting off at your stop (which isn't uncommon during peak times), ask someone to press it for you, or if nobody can hear then force your way across to the button.