A bell is helpful in a situation where a pedestrian is not aware of an approaching bike. The bell informs the pedestrian of two things:
1. That there is an approaching bike.
2. Roughly were the bike is approaching from.
The hope is that the pedestrian will then behave in a predictable way to allow a safe pass by the bike. In almost all cases the pedestrian will be able to simply continue doing what they were doing before they heard the bell.
If a pedestrian can not hear bike bells, for whatever reason, that is not a problem. They can just stay consistent with the centreline of the path/road/way. They then have a responsibility to shoulder check when shifting from side to side.