I thought it was an access hole to get to the cables inside the seat tube, but when I unscrewed the square black plastic plate, it's just a flat bare metal surface, with a vertical slot cut in offset from the centre.
Is this for a chain guide?? Seems too high for that. I can't find anything in the user manuals on Giant Trance (this is a 2018 Trance 2).
It’s to attach a direct mount front derailleur if you want to run a 2x or 3x setup.