HT is probably one of the most undervalued attributes in GURPS, in my opinion. It is described in the Basic Set (p. 17) as “energy, vitality, stamina, resistance (to poison, disease, radiation, etc.), and basic ‘grit.'” I take that last part as “physical” grit – because grit isn’t just physical (or mental). To get a gauge of that you’d need to figure out the average of HT and Will. A strong mind can overcome physical limitations, and vice versa. You need a good HT to survive the various physical rigors that the GM can put your character through, and it also boosts Basic Speed and Move – it’s essential for most “low-tech” characters. But most of my players buy at least an HT of 12, and unless it goes against their character concept, Fit and Rapid Healing too. Given the typical length of my campaigns and how I build worlds – these are good buys. It lets them keep their original characters longer. In fact, the only two original characters remaining from the beginning of one of my first campaigns had both Fit and Rapid Healing. This sort of set the precedence for most other characters in my other campaigns. High HT and both Fit and Rapid Healing, and some level of Regeneration when allowed.
FP even more important, especially since most campaigns I run tend to have some set of supernatural ability and fueling them comes off FP. But FP isn’t just your mystical battery. It’s a measure of physical stamina. How long you can run, how long you can fight, basically, how long you can last at any given physical task (keep your mind out of the gutter!). Add that to the fact that I charge FP for most physically exerting tasks (and the HT roll to go with it), and you get another popular “dump stat.”
Though I do run cinematic games for nearly every campaign (my player’s choice, not mine), I tend to use the realistic rule set and then let PCs purchase Rules Exemption perks for those they want to ignore. Bleeding to death even when you might be one of the greatest mages in your generation is quite dramatic, which has the same connotation as cinematic to my players. They love it. They want to get beat up, bruised, battered, and bleed for their cause (whatever that may be). High HT and FP let them do this.
Continuing with the analogy: HT represents how resistant you are to any diseases or pesticides an apple may carry, how many times you can climb an apple tree and not become fatigued, etc. FP represents how long you can pick apples, how long you can dig holes to plant apple trees, and so on.