Two major problems associated to nearly all games relate to replay value and playability. Replay value is usually determined by how much more involved the player can be in a game outside of it's base experience, where as playability concerns the amount of fun a player can have without feeling bored or unsatisfied. When constructing a game built around preset values (non-progressive integers and mechanics), both replay value and playability are called forth to the table a lot more.
The most interesting point to know is that most features in DLC and game updates relate to expanding content; in other words, longevity. Procedural content generation in VHEL does just that: build a create new content from preexisting assets within the game without a designer (me) telling the game what to do.
Ordinarily, game world environments are crafted by hand for specific design reasons. The issue in having preset environments and objects, however, relates to player ingenuity. The typical player is capable of remembering every facet to a map, no matter the complexity, while also coming up with simple solutions to overcome otherwise difficult odds or challenges. In time, preset maps can dissolve into predictable "go here, then there" race tracks with difficulty curves diminishing greatly.
As so, VHEL has procedurally generated environments (like the title says -- duh). Unlike the redundant random dungeon generation trick in some games, environments in VHEL are determined and built around several factors: average stamina and stress ratios, the play style in which the player favors (cloak, cunning, or combat), how much of a map the player covers over a given amount of time, how often the player uses certain items/strategies, etc.. From here, the game dynamically generates new areas around the player's strengths and weaknesses. This can range from strategically placed bushes and elodea fields to force the player to traverse slowly, or limited cover points encouraging the player to push forward.
In many games, items range from "horrible" to "so good it's irreplaceable". Due to this, item scouring and anticipation for the next best thing can diminish over time. In VHEL, like it's environments, items are generated automatically with largely different stats, attributes, and effects. This means that EVERY item within the game can still retain some form of value since newer items can be found almost immediately with alluring features.
Procedural Enemy AI
With enemy AI, there needed to be balance between preset and "random". Difficulty curves are a crucial element for player progression and development, so whimsically having enemies pop up all over the place was not ideal. Preset enemy placement, obviously, is not ideal either. If you have ever played L4D, then you know already where I'm going with this. For those of you who haven't, play it.
In short, dependent on how you play, etc.,etc., the game will adapt to you and change up which enemies will come after you and when. As the game progresses, so do the enemies and their general tactics, abilities, and ways of dealing with you. The nice part about having such a system is that it merges seamlessly anywhere at anytime, and works hand in hand with the dynamically generated environments.