The Scout is optimized for the anti-air role in a similary way to the terran wraith. In comparison, the Scout has greater durability but no cloaking device, and is slower without the Gravitic Thrusters upgrade which gives the scout equivalent speed. Due to the explosive damage of their anti-air attack, they have strong anti-air attack to counter heavily armored flyers such as battlecruisers. Their ground attacks are fairly weak. Ground anti-air units such as the goliath or hydralisk counter the Scout. The Valkyrie's diffused attack is less effective against lone Scouts.
The Scout is rarely used in competitive play due to its uneconomically high cost, the aforementioned weak ground attack, and the paywall the Gravitic Thrusters upgrade represents. Instead, niche roles are filled by the corsair.
However, the Scout is more useful in of the Protoss campaign. They are first used in Higher Ground against Mutalisks, although they really shine against other protoss units in Homeland where their explosive-type missiles do full damage to shields and dispatch enemy Arbiters and Carriers. However, for balance purposes Scouts are large units and take full damage from Dragoons.
The Scout, or at least its model, was originally a speeder bike. A protoss rider was visible on the unit. However, the tab was removed as it wouldn't fit the other protoss units, especially in terms of size—comparison has been made to the battlecruiser, where what would be a single protoss rider would look like a giant compared to a capital ship.
In the StarCraft alpha, Scouts were armed with disruptors. In the StarCraft beta, Scout engines produced green exhaust instead of their current blue. Scouts also used their anti-matter missiles against ground targets.