In the earlier editions of D&D--and the current retroclones/simularica--the management and attrition of hit points are the core element of the game (more about that in a later post). The ability to avoid, withstand, and inflict hit point damage forms the most fundamental aspect of the game.
A key facet of hit point attrition is the ability to increase one's own hit points and also increase the ability to inflict hit point damage. This, of course, is done through moving up in experience levels. This, in turn, is the resulting of gaining or earning experience points through play.
Giving the importance of awarding experience, I am baffled at the lack of attention this gets in the many rule sets. When I was playing Castles & Crusades, I was at a loss as to how to award experience. Well, not really at a loss, I just used my systems from my AD&D 1e days. When I started using Blood & Treasure, I discovered that the guidelines for awarding experience were not included . While that is a publishing gaffe, I have been using the x.p. awards for monsters overcome plus 1 x.p. per gold piece of treasure (nothing for magic items). I plan on tweaking that as my first "official" house rule for the Montporte Dungeon campaign.