At the moment, going on dates with your companions will help improve your stats and unlock new skills/spells.
In terms of erotic options, Morrigan is the most fleshed out. Depending on what you say to her, you have some unique scenes/choices that aren't otherwise available to you. Tera also has a few scenes attached to her storylines, but there isn't anything for Celine just yet, as hers is intended to be more of a slow-burn romance.
The dating system currently in the game was mainly put there for testing purposes and will be revamped in the coming weeks when we focus on polishing up the old content