The longer the distance an Egg requires, the rarer the Pokémon will be. Although it may not seem like it when you get a Paras from 5KM egg or an Eevee from a 10KM Egg... more often than not the longer the distance the Egg, the better the Pokémon!
Pokemon Egg Hatching Chart (2KM, 5KM & 10KM)
The longer the distance an Egg requires, the rarer the Pokémon will be. Although it may not seem like it when you get a Paras from 5KM egg or an Eevee from a 10KM Egg... more often than not the longer the distance the Egg, the better the Pokémon!