Players see through their default vision when they control a token without vision
In core Foundry, when a player selects a token with "Has Vision" disabled, they can only see their previously explored portions of the map through the fog of war. They can't see other tokens, including their own, because they are hidden by the fog of war. This is very inconvenient for cases where a player has summoned a creature or token, but you don't want the player to be able to see through that token's vision, such as a spiritual weapon or the like. Previous workarounds basically required giving the token vision, which could result in spoilers or simply a tactical advantage from seeing around corners.
No Summon Vision to the rescue!
Now, summons can have "Has Vision" disabled, but the player will still be able to see through their default view—the union of the vision of all of their tokens with vision.