You should catch common Pokemon in Pokemon Go because they are a great source of XP points and candy. Even after you have evolved your low level duplicates (e.g. Rattata into Raticate), you should continue to catch the common Pokemon so you can evolve them for XP points.
Also, when you begin playing the game, the only way to evolve your Pokemon is to catch the same common Pokemon repeatedly. After you have caught enough of the same Pokemon, you can transfer your Pokemon to the Professor for candy. You will need these candies in order to bring your Pokemon to the next evolution state.
There are a bunch of common Pokemon that you will find very frequently. Here are a few of the low level Pokemon that you will run into frequently while playing the game:
- Nidoran (F)
- Nidoran (M)
Common Pokemon will vary depending on where you are geographically. For example, if you are in an urban area you will find more Rattata and Magnemites. In more rural areas you will find more Eevees.