Because Pelliper's abiity Drizzle makes it a staple in rain teams. Due to It's typing, it can check Landorus, keldeo and MegaZard X easily.
It has a wide and good movepool. It can defeat Toxapex with Choice specs. Damp Rock makes Drizzle lasts for 8 turns and Drizzle lets Mega Swampert, Ash-Greninja to sweep. These are the only useful things I can find.