It's better when it's dirty
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.
I can't say for sure, but likely a front derailleur mount or for a drop cage. That looks like the right height for a traditional double and smaller cassette. The frame probably comes in a lower spec variant with cheaper group sets or was planned at earlier stages of production design.
Front derailleur mount.