Like in Pokmon? Cool concept but they implemented it in the worst possible way.
Itd be cool if theyd used them to give focus to forgotten lines or, even better, to power up unevolved versions of Pokmon so that thered be a reason to keep a Wartortle or Quilava on hand.
Instead they choose to give them primarily to already powerful, popular final-stage Pokmon, some of which dont and now cant have evolutions. Not to mention, that almost all of the Pokmon they chose were already designed to be final forms, so adding random nonsense makes them feel extremely over-designed.
And to put the cherry on top, they immediately abandoned the gimmick the following gen in favor of z-moves (which also seem to be getting abandoned this gen), so Pokmon that should clearly have Mega forms likely never will.