We have the lingering issue of air leakage at the window and door. The interior trim is an attempt at tightening up the seal at each.
It also helps keep the insulation attached, and adds a bit of style (I also added a thermometer).
My younger helper and I also caulked a bit. There were gaps at the corner posts, and we also caulked along the edge where the tile meets the floor studs, and at the door and window.