DAS uses more sensitive heat detectors, which can spot a ballistic missile being launched over 1,200 kilometers away or another aircraft 30 or more kilometers away. Recent tests discovered that DAS could detect ground fire (tank guns, artillery, or rocket launchers). This was an unexpected bonus, as it allows DAS equipped aircraft to quickly identify these weapons on the ground. If any of these are enemy systems, they can be immediately attacked with smart bombs because DAS can also calculate the GPS coordinates of what it spots on the ground.
The main purpose of DAS is to spot aerial targets without revealing yourself by transmitting a signal (as radar does). This is called passive sensing and enables the user to get the first shot in with a missile. For obvious reasons the air force did not reveal what the exact detection ranges were.