At the bottom right of the playing screen, there is a small banner that displays 3 Pokémon. Selecting it brings up a screen that details all the nearby Pokémon.
Previously caught Pokémon show in full colour, uncaught Pokemon show as a silhouette, and doubles can be displayed. When I select any of these Pokémon and click the footprints the Pokémon becomes circled in purple on the previously mentioned banner.
Why would I want to "watch" a Pokémon in this way?
I have observations and assumptions, so far, but nothing concrete.
Best Answer
There are two things to look out for when hunting Pokemon.
1) The number of feet - the number of footprints drops from 3-2-1-0 the closer you are to the Pokemon.
2)The Pokemons position on the nearby pokemon list (or Poketracker as i refer to it). Top left is closest to you, bottom right is farthest.
When I am hunting a specific Pokemon I keep the Poketracker up , if the silhouette moves left I know I am getting closer and if it moves right I know i am getting further away. Think of it like a very sophisticated version of marco polo or Hot&Cold.