Not just pedestrians, slower riders also, would like some warning you are passing.
Actually, although the law/rules are pretty clear in most places, it is just courtesy to look out for others in a crowded place.
Unfortunately, there are some cyclists, and operators of other vehicles as well, who just don't get the ideas of courtesy and safety. They are the ones who pass without warning and so close the draft from their arms can be felt. They are the ones who ride a pace line at 20mph through a crowded park. They are the ones who ...., well you get the idea.