Vision shows how far a unit can see and clear fog of war. This doesn't detect stealth units.
Usually units have higher range than vision. But there are exceptions, such as artillery units have very small vision and can't attack ground to fog of war, so use scout units to assist them.
The following units have smaller vision than range:
Commander abilities can also be used only where the player has vision. Vision is shared across your team members, in bigger maps these are only 3 players.