good example of this Leafeon Lv X and Lanturn prime. you can evolve into Lanturn from the bench, but you can't level up leafeon from the bench unless you use Porygon or level max. and they both have the same HP and basic type of attack. leafeon's poke-power lets you attach more energy, but using Lanturn with Gatr lets you use Lanturn's poke-power to attach even more energy. also Lanturn is stage 1, while leafeon is more like a stage 2 with stipulations but more attack options.
Primes really were a way to replace lv X, and that example is almost an exact copy to compare it to.
fire decks will run typhlosion prime, water will run gatr prime, leaf will use meganium prime (sometimes), electric will use magnezone prime, psychic will use gengar prime as well as maybe mew prime, fighting will be some sort of machamp/prime and donphan prime variant.. Prime's just have great abilities (most of the time) that are too good to pass up. there really are some terrible primes out there where as there really isn't a bad Lv X out there, but compared to the non prime versions a prime will undoubtedly be better, especially if it's within the same series.