Evolution calculator tells you how much CP your evolved pokemon will have, so that you can consider whether evolving a certain pokemon is worthwhile.
The best strategy seems to be to collect many ready-to-evolve pokemons, then start a lucky egg and evolve them all in a batch. This way you get double XP and so you progress to the next level faster.
However if you're really tempted to evolve an odd pokemon here and there, then this calculator is a useful reality check. Will the evolved pokemon be worth it? Or is it better to wait and evolve it later?