A target scope can be as simple as a laser sight, but most are elaborate versions linking your weapon to a visor or goggles.
Weapons are never more accurate than your posture allows. Firing a weapon at long range requires that you stand still - otherwise the range can at best be normal (20 meters).
With a target scope, this restriction is lifted. You can fire hip shots, and the targeting scope still lets you aim accurately, letting you use a weapon with long range even while moving.
Laser sights are cheap and common - free of charge. But they have a visible signature, and thus warn your target that you are aiming at him. It is also highly confusing when several laser sighs play over the same area, so if more than three people are using laser sights at the same time against the same target area, none of them works. Paying an item schtick for a proper setup avoids all these problems.
It is a one-shot action to turn it on or off.