I think bells do have a communication use of course, just not really to be used as an emergency 'an accident is about to happen, immediately take action'.

At least a bell sounds relatively polite if you're not spamming it. A horn is a bit aggressive, you have to modulate it.

In a car I use two short tapped toots as a polite kind of 'excuse me' e.g. if someone hasn't noticed a light turning green. That seems more friendly than a sustained blast.

On the bike with a bell I'll just say thank you as I pass, if they've moved for me. Usually seems to go down well enough.

it's a good habit to just ring your bell when approaching things like merges and city intersections regardless if there's other people; you tend to do it earlier and might miss seeing someone.