It seems like Niantics can’t really make up their mind about how their users are going to tracking down Pokemon. When the game was first released, it came with the glitchy footsteps tracking system that was later scrap. We were left with the even more confusing nearby features and now, Niantic has replaced it with the new Sightings.
With Footsteps, you got to see how far you are from the Pokemon just like nearby where the ranking will change base on how close you are to the Pokemon but Sighting will offer you nothing except the knowledge that a certain Pokemon is nearby.
However, some users are saying that it is much more helpful as it refreshes faster which means it is constantly updating you on the Pokemon nearby so that you can estimate their position. The search limit has also been reduced to 200meters.
We don’t really feel that this is an improvement but we know that Niantics is working on an update for the new Sighting feature. Hopefully, they will get the update ready soon.