With each level of this Perk, you gain an additional 4 Hit Points every time you advance a level. This is in addition to the hit points you already gain per level based off of your endurance.”— Fallout 2 description
+4 hit points per level gained (+8 with both ranks). The gain is quite substantial and with both ranks effectively doubles the HP gain at 10 Endurance could give (8 from the perk and 7 from the stat), giving an excellent margin for any heavy fighting and criticals. Even a single rank is a substantial bonus.