Scouting is essentially the action of determining where an enemy is, and what weaponry and units he has there. This information is vital to making a successful attack. A unit must have several qualifications before it can classified as a light scout unit.
1. The unit must be fast. Slow units can't cover enough ground to act as scout units.
2. The unit should be cheap and therefore expendable. Scout units that run into enemy patrols or bases tend to be quickly destroyed.
3. The unit must have at least an average sight range. Although Technicians from the Tiberian Dawn era meet the previous two criteria, their lack of a sight range prevents them from acting as effective scouts. Aircraft have a particularly large sight range.
The following are helpful to have in a scout unit, but not required.
2. Ability to detect stealth units / hidden units. If the enemy cannot hide from a scout unit, then the scout unit can provide additional information.
3. It can also be helpful if it is a stealth unit. Scouts are usually fragile and lightly armed, but this is somewhat negated if it is invisible, as it can hide away from enemy radar.
4. It also should have anti infantry weapons to kill patrols
Nod's Venom aircraft are an excellent example of a unit that combines speed, the ability to cross any terrain and a large sight range, along with being particularly cheap and easy to construct in quantity. (However, as with the majority of scout units, they are also fragile.)
In short, light scout units are units specifically designed to gather information about the enemy and his environment. Orca Gunships have sensor pods to detect enemies and might attach to them.
But there are other ways to find out what the enemy is hiding. Light scout units are only optional in modern warfare since technologies like the spy satellite, intelligence, or other technologies are invented to freely scout the terrain.