Harp SF as a game was split into two books to keep the page-count of the core book lower. Harp SF Xtreme are those parts that was split out to its own book. These are rules for vehicles, vehicle combat, cyberware and electronic characters. In some sense these are more narrow areas, for a large gaming group it makes sense to have more copies of the core book around than this supplement.
Another point of view would be that this book covers parts of the original game design that was so expanded with player feedback that it could not fit in the main book. If you are looking for original content, what makes Harp SF different compared to other games in the market then the majority of such content can be found in Harp SF Xtreme.
A simple example would be the name of the setting, why the setting is named after the Tintamar space station is not very clear after reading the main book. When you add the vehicle rules, especially how hyperspace travel works, it becomes quite clear that the Tintamar space station is pretty much the center of the setting in the gaming universe. It is the natural staging ground for long range space expeditions and it is also the choke point that the enemies of the human race need to conquer if they want to do serious battle with the human race.
Personally I think the move to split the core game into two books was a mistake. At the core games like Harp and Rolemaster are about challenging monsters like a dragon or a demon. These are compared to much else very rules heavy games, but the detail of the rules mean the battle with monster is interesting and worthwhile on its own. From the main Harp SF book it is not very obvious why to prefer this game over the Harp Fantasy game. There is no obvious dragon to battle and the technology is some senses quite low-tech compared to other SF games and more futuristic settings. Harp is a competent game engine, but no real answer is given on why the effort was taken to export this rules set to science fiction.
I would say the answer to this riddle is pretty much hidden inside Harp SF Xtreme. Harp SF is meant to be a game when a group of heroes work together to do space combat. The comparably low-tech science fiction ties directly into that space combat should not be a one man affair, but be an effort of the whole group. The clever hyperspace rules invite for a game of exploration and drama as heroes stumble on enemies and can not just jump to safety directly but must find a way to stay alive until they are in a position to return.
If you are looking for a game when the "Tricorder" and some technobabble solve everything Harp SF is not something for you. Harp SF gives you plenty of gadgets to play with, but mostly it is a science fiction game about the characters and how they struggle to stay alive. The GM is given a massive set of tools to realize the "High Action Roleplaying" part of the Harp name.
All in all I think Harp SF Xtreme is a purchase that might give new inspiration how to run more gritty space combat also for people who don't play Harp SF. I think it is a essential purchase.