You don't need a pokewalker. However it is used to catch Pokemon and find item , raise happiness of a Pokemon, slowly level it, and can be quite fun when you are some place you cant play your DS.
Some Pokemon can only be caught on the walker, however you could always trade people for them as well.
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The route to find a sneasel in the poke walker is called Icy Mountain Road. You need 30000 Watts to get the route. You also need to walk 3000 steps on the poke walker before being able to encounter sneasel on the route.
I believe the Pokewalker requires a very small Phillips head screwdriver.
The only way to get a Poke Walker is to buy Soul Silver, Heart Gold, Diamond, or Pearl.
you cant
poke walker route: tree house