1) If your stock strap is 24/22 (24mm at the lugs, and tapering to 22mm at the buckle) and you buy a strap that's 24/24 (non-tapered, like most aftermarket straps) then you need a new 24mm buckle. If your stock strap is 24/24, or have a stock 24/22 are ordering a replacement that is also 24/22, then you need nothing.
2) They replace the lug screws that hold straps in. Good replacement for watches where the screws or the holes for the threading are stripped, and also good if you're like me and change straps 2 or 3 times a day.