The benefit of airless spraying over all other forms of paint coating application, is that no or a very minimal amount of thinner is added to the paint.
Airless spray is quite simply paint applied at great pressure, atomising the paint prior to pushing it through the nozzle.
Applying primers through airless spray helps with penetration and even distribution on inaccessible and awkward areas.
A further advantage is that as there is no air mixed with the paint, there is no danger of having wet air, which would then contaminate the paint being applied.