Concealing the map beyond a player's vision is the single design choice that most changes how a strategy game plays. It converts calculation into inference.
Perfect information produces a solvable problem
If both players see everything, each can evaluate the opponent's position exactly and choose the best response, and so can the opponent.
The game becomes a matter of computation rather than judgement, and the better calculator wins consistently. Skill collapses toward a single dimension.
Hiding information removes the ability to compute a best response, because the inputs are unknown and must be estimated.
Absence of evidence becomes evidence
A player who scouts an area and finds nothing has learned something: the opponent's forces are elsewhere, and the possibilities have narrowed.
Skilled players track these eliminations continuously, building a picture from what has not been seen as much as from what has.
This is a distinct skill from executing a plan, and it is largely why strategy games reward experience so heavily. The reasoning improves with exposure.
Concealment becomes an active strategy
Once information has value, hiding it does too. Players deliberately conceal production, misplace forces to suggest intentions they do not hold, and destroy scouts to maintain doubt.
These actions cost resources and produce nothing directly, which means the game must value information highly enough to justify them.
Fog is what creates that value. Without it, every resource spent on deception would be wasted.
Different fog rules produce different games
Some designs reveal terrain permanently while hiding units, so players know the map and not the opponent. Others hide both until explored.
Games that show the terrain focus attention on the opponent's decisions, while games hiding terrain add exploration as a separate concern.
Rules about remembered information matter too. A game showing the last known state of a location encourages inference from stale data, which is a different skill from acting on current vision.
It creates a spectating problem
Watching a match is far more interesting with full visibility, since the audience can see the misunderstandings that the players cannot.
This is why competitive broadcasts show an observer view with all information revealed, which is a different experience from playing.
The gap explains why viewers sometimes find professional decisions baffling. The player was reasoning from a fraction of what the viewer was shown.