OK, there are two different things. One is the actual PokéRadar, the other is the PokéTch app that comes along with it.
When you're inside a patch of grass, use the PokéRadar to detect Pokémon in certain patches. That's all it does. However it's said that you have a higher probability of obtaining shinies this way. It is also the only way to get some Pokémon exclusive to certain areas, etc.
The PokéTch app tells you the top Pokémon you've encountered, through the number of times you've seen that certain Pokémon. It's not essential for your hunt, but it's nifty if you want to see which one to expect the most.