Missiles are highly specialized units, able to hit Air units from range. They can be considered to be the equivalent of Rockets versus Air units, but tend to be less effective due to Air units having better movement.
Missiles are designed to destroy Air units, either instantly destroying or dealing very high damage to any Air units that venture into its range.
In typical games however, this rarely happens, as all Air units have higher movement ranges than Missiles have firing ranges. Combined with its low movement speed and bad movement type, Missiles are extremely easy to outmaneuver with Air units.
Despite this weakness, Missiles can be used to deny Air unit attacks on a group of units, as long as the Missile is well protected and the units are all within Missile range. Unlike Anti-Airs, they do not expose themselves to possible Tank counterattacks when attacking Air units, so they have a place, albeit very niche, in a formation.
Missiles are often used to effectively prevent an Airport from being able to build units by moving it into range of the Airport. Generally speaking, however, if the opponent allows this to happen, the game tends to be already won, as Airports tend to be relatively close to other production facilities.
Missiles are often a very expensive invesment for defending against Air units, and usually leave not much funding for building other Land units. With this in mind, one can start trying to build more Land units if they see the opponent build a Missile.
It is often good practice to make sure to check Missile range before moving Air Units to avoid unnecessary casualties.
Missiles take high damage from all units, including Infantry and Air units, so an exposed Missile is easily destroyed. Missiles are easily avoided however, so it may be safe to ignore Missiles unless you need to open the area for Air units to travel.