Heroes that use Health as a resource for activating their skills. A skill is not usable if you do not have the remaining health to activate the skill.
Some Health based skills take a percentage of your remaining or maximum health in order to activate, while some skills take a fixed amount of health (e.g. "30 health").
Buying items which increase your health regen would significantly increase the usability of these skills, while buying flat health would only significantly increase the usability of skills that require a fixed health cost.