Energy Is A Type of Store Currency

Many games have Energy systems. 

The biggest Collectible Hero / Squad RPG games like Marvel Strike Force are based around Energy. One comment I often hear is that a game like that based around Energy system has more player directedness and agency -- you enter into a session figuring out what you want to spend your Energy on. Usually on furthering some sort of PVE instance or farming a node for materials/shards.

We can really think of this as: Energy is really another currency with the various PVE instances or Campaigns serving as Storefronts that take Energy in exchange for materials/shards. Yes, there is RNG and yes, there are failure conditions, but at heart it's a transaction.

Some unique properties common to most Energy games:

• Energy is a resource that accumulates through time
• All players accumulate Energy at the same pace
• All players have the same Storefronts (Campaign/PVE nodes) to spend Energy on
• Players spend differently based on their goals 
• E.g. Player P1 spends on node 42-1 to get resource R1 and Player P2 spends on node 10-3 to get resource R2
• Goals may be different based on RNG or point of lifecycle, but the most interesting would be enabling the player to make meaningful choices that branch 
• E.g. A player farming node 10-8 for shards of Character C9 as she wants to power it up for a specific comp, as opposed to spending it on the various other nodes that offer other Character shards, and she never will farm those other nodes in her game life

That being said, I think that property of agency is missing in some games out there, particularly those which don't design the "Energy Storefronts" in meaningful ways and/or games without Energy. 

Assuming that player agency drives engagement, which results in better retention/monetization. 

My intuition is that in games without Energy that end up in a low-agency state, you can make up for some of that lack by implementing more systems that allow for choices per session per day. E.g. Creating features that have space for meaningful player choice like the rogue-like Labyrinth in AFK Arena, the various Stores with different currencies (though these are often not balanced for daily/per session decision making) and so forth.