I use them mostly in places to give people a quicker alternative. for instance, if the commercial zones are right next to a large suburb, but there is no direct road connection between them, I would tend to squeeze a footpath in there for easy access. I also realized that having the road crossings mod is crucial to have these work in an optimal way, especially when you want the path to cross more than one road.
I also like using them to make my open areas look like parks.
I don't overuse them though. If people can get where they want using the normal sidewalks, there is no need for additional walkways.