Regarding your question on the `footway=sidewalk` ways ("Is this detail necessary?"): It's not strictly _necessary_, but it is helpful.

In general, the more detail that can be added, the better. It doesn't have to be comprehensive, but each little improvement adds up.

For footpaths ("sidewalks") specifically, more detail can be helpful for accessibility routing information — e.g. https://www.opensidewalks.com/ and https://wiki.openstreetmap.org/wiki/Guidelines_for_pedestrian_navigation