I don't see the XGT platform as a great option if you mainly want an impact driver. Yeah, they have one on the platform, but that is a convenience for the folks using XGT for the higher power tools, not really a reason to buy into the platform.
When you see the driver alone that is a "bare tool" (model number ends in "Z"; e.g., XDT16z). Kits can either be a single tool with batteries and a charger or they can be "combos" that include multiple tools, batteries, and a charger (sometimes accessories also). If you need the tools anyway and don't want to shop around a ton, combos can be a good deal. For example, I'm not sure if the deal is still going, but recently HD had the top tier impact driver (XDT16) AND top tier hammer drill (XPH14) in a kit with four 5ah batteries (2 in the kit plus two more on promotion), a charger and an XPS bit set for $399. While expensive overall, it is much cheaper than buying all the pieces separately.