The zerg have a unique capacity for scouting: their supply units (overlords) can double as scouts early in the game. Later on, they can be upgraded to carry units, or upgraded to move faster.
Additionally, the zergling is significantly faster and cheaper than any of the other early-game units. By moving a small pack of zerglings around the map (and controlling Xel’Naga watchtowers, if they are present), you can ensure that you’re always aware of your opponent’s moves, even if you can’t see inside of their base.
Zerg players also use creep to provide vision across the map.