Because there are some moves that it can only learn in it's current stage or just moves that it learns quicker in it's current stage. And of course becaue some people just like their original Pokemon better. :)
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Give them a everstone or press b when they are evolving ;)
The everstone stops evolving all Pokemon that evolve in the first place.
There are two ways to stop a Pokemon from evolving. The first is to give it an everstone, which prevents you from working everytime the Pokemon is ready to evolve. The second way is to press "B" when the Pokemon is evolving. If you did neither, then your Pokemon is not ready to evolve.
If you stop a Pokemon from evolving, it`ll try to evolve every level after that unless it`s holding an everstone.
If you accidentally pressed "B" while your Pokemon was evolving, all you have to do is level it up again.