That would depend entirely on the game. For example, the game Cassette Beasts has multiple DLC. One adds a small amount of actual new content to the game, and the rest are all cosmetic outfits for your characters. None of these are worth buying unless you really like the base game and want to support the developers. Then there’s games like The Binding of Isaac: Rebirth, where each of the DLC is a massive expansion to the base game, with the DLC having more content than the base game does at this point and are highly worth getting.

As far as Trails in the Sky is concerned, I know some of its DLC is cosmetic stuff and a lot of it gives you packs of in-game consumables. Idk if there are any expansions planned for it.