Yes, sealing a garage door threshold will improve insulation. For starters, the strike and plate of any garage door are usually metal, which significantly increases heat transfer. To improve insulation, you can install a new polyurethane seal on the in-ground piece of the garage door threshold. This will cut down on some of the heat transfer. Installing a new door seal along the base of the door which not only covers the outside but also the inside of the door, as well as the track in between the sides of the door, will also improve insulation.