I would say the point of LC is that every Pokémon in the tier can evolve, yet they stay their base forms. Eviolite is also necessary to balance the metagame, since common LC sweepers such as Abra would cause a lot of chaos if there weren’t things that could take a hit from it, such as Eviolite Munchlax.
252 SpA Life Orb Abra Psychic vs. 252 HP / 4 SpD Eviolite Munchlax: 12-16 (36.3 - 48.4%) -- guaranteed 3HKO
252 SpA Life Orb Abra Psychic vs. 252 HP / 4 SpD Munchlax: 17-21 (51.5 - 63.6%) -- guaranteed 2HKO
I mean, Munchlax OHKOs Abra with Body Slam, but if a Munchlax has taken previous damage, the Eviolite could help it survive a Psychic.
So, really, it’s all about adding balance to the tier, and, of course, it’s an item specifically meant for NFE/not evolved Pokémon, so it makes sense. Without Eviolite, LC lacks bulk, and slower, mostly defensive Pokémon would be rendered mostly useless (not saying they would be completely useless, but the boost from Eviolite helps a lot) because their stats just can’t match up to those of a LC sweeper’s.
Hope this helps! :)