Which locations are worth using a pharros stone? [closed]

I already encountered the pharros slot in the Wharf (which I found quite useful), the one in Earthen Peak (which wasn't so useful) as well as the massive amounts of pharros slots in the Grave of Saints.

In what locations is the use of a pharros stone tied to a welcome result? Getting to know the game, I don't assume to be able to use all slots in one playthrough...

By "welcome result" I aim for results like exposing items, leading to hidden paths or decreasing the difficulty of the game (like in the Wharf).

A full list of Pharros stone rewards, this will give you all the information needed. I tried making a quote but that would simply mean copying the entire page. As useful is hard to describe you can determine yourself which you want to use.

I don't know if Scholar of the First Sin has any additional.