Big crap

When you catch something huge, even as the base type of a pokemon, it's more often than not worth it to keep it no matter its associated IV.

800 plus eevees, 1400 rhyhorn, 2700 snorlax, etc. You basically get a shot at an instant defender for a crap gym.

This is mostly attractive if you, for some reason, place pokemon into an insane number of gyms. Because you're unlikely to have two dozen grade A defenders, and for that reason it's nice to have access to a decent number of 2500 plus CP big crap.

A full raid should max out half a dozen to maybe ten gyms. They need to be populated, even though you're likely to get your pokemons returned within a day or two. A weekend spent on raids is likely to deplete your collection of pokemons that you're interested in dropping into gyms.

So, big crap comes in handy. They're basically free pseudo defenders.