my own experience is that in the city the bell was to alert people that I think aren't paying attention to me and may be about to step into the bike lane. 100% like you said, I'm letting someone know I'm there

Now that I moved to the country with a comprehensive rails-to-trail network, I thank all the cyclists that use the bell to let me know they're coming up behind me. What really irks me is the dudes going 30+mph silently coming up behind me, passing less than 2' from my dog (who is at my side) when there's PLENTY of room to give me space. No, we can't hear them coming all the time. Yes, it's startling, rude, and dangerous for all of us.