Tiers are used to keep competitive battling balanced. If there were none, people would be using Arceus and other Legendary Pokemon in every battle, and that wouldn't be fun now, would it?
Tiers:
Overused (OU) 56pkmns
Borderline (BL) 4pkmns
Underused (UU) 57pkmns
Borderline 2 (BL2) 3pkmns
Rarelyused (RU) 59pkmns
Neverused (NU) 203pkmns
Little Cup (LC) 212pkmns
Limbo 1pkmns
Not Fully Evolved (NFE) 51pkmns
Source