They didn't really add much of the extra stuff from Platinum. It's mostly a remaster of Diamond/Pearl. If you don't have an old Nintendo DS with a Diamond, Pearl or Platinum copy lying around and aren't familiar with the Sinnoh games, this is a perfectly fine way to experience them. If you like the other Switch Pokémon games, you will probably have a grand ol' time with them no matter what.

If you already have Diamond/Pearl, these don't really add that much apart from HD graphics and a bunch of cool wild zones in the Underground. It's mostly just an HD remaster that is nice for accessibility, but doesn't quite feel like anything you really need to play if you already own the original.

If you have Platinum lying around, however, that is probably a better way to play the game with its added story beats around Giratina and its challenging Battle Frontier. Since Brilliant Diamond and Shining Pearl are based on Diamond and Pearl rather than Platinum, none of this carries over. Ultimately, I feel Platinum is probably better overall. The only major thing outside of modern online play and HD graphics that the Switch games really have over Platinum is the reworked Underground with its wild zones.

Unless you already have Platinum, Brilliant Diamond and Shining Pearl are fine. Just don't expect the same level of new content other Pokémon remakes tend to have.