Well, two good reasons come to mind: it is the least costly of the three primary methods brush, roller and spray and it is the fastest application method. The more irregular the space being painted, the greater the advantage of spray over other methods.

An airless spray rig consists of the pumping unit, hose and a gun. No compressor is needed because the paint is pumped through the high pressure hose and forced thru very small tips which break up the paint into various size spray pattern fans. The biggest advantage to airless spraying is that since no air is used to force the paint out, there is very little over spray and the air in the room is not filled with paint laden mist.
The amount of paint delivered is determined by the size of the tip used and by the amount of pressure used. Tips normally are numbered in thousands of an inch.

Example: .019 tip indicates a somewhat large tip suitable for most latex paint, a .015 would be ideal for oil base paints, enamels and varnish coatings. Insulating ceramic paints spray best with tips in the .021 range, be sure and remove all filters when using ceramic paints as you do not want to filter out all the ceramics.

CAUTION: The pressure at the tip is normally in the 3000 psi range and it can not only cause a severe cut but also inject paint into your skin! Keep your hands and fingers clear of the tip while pressure is on the machine!

Tools and equipment you will need include, A 5 gallon bucket to mix all your paint together in, (boxing), to insure you have the same color throughout the mix. Masking tape and lots of plastic to mask off areas not to be painted, A respirator or spray mask, A hat, long sleeve shirt, and some hand cream or lotion for your exposed skin.
When you pickup your spray unit from the paint or rental store ask for an "extension" for the gun. These simply screw onto the gun allowing you extended reach and eliminating ladders.

Procedure: Mix all your paint into the 5 gallon pail, insert the dip tube from the pump into the bucket or fill the hopper depending on which unit type you have, make sure all your fittings are tight.
Now, all units have two control knobs, a pressure control and a pump/recirculate knob. In the recirculate mode the paint simply recirculates thru the unit, pumping out trapped air and allows the pump to prime. Turn the knob to recirculate and turn on the power switch. Allow the pump to run for 2 minutes or so and it will prime and push out any trapped air. Now we are going to adjust the pressure. The biggest mistake most people make, even some professionals, is spraying with too much pressure.

You need just enough pressure to create an even fan with no "tails" which are thick lines at the edge of the spray pattern. Back out the pressure knob and screw it in about 1/3rd of the way (screwing in increases pressure), point the gun at the surface to be painted or a piece of cardboard for testing and with your hand in motion, pull the trigger. If you see a thick stream at the edge of the fan increase the pressure 1/4 turn at a time until the tail is gone.
Now you have the proper amount of pressure for the material you are spraying.
Technique! Every time you pull the trigger you must follow the same routine. Remember that the second you pull the trigger the paint comes out instantly so, start your hand in motion first and then pull the trigger, release the trigger just before you reach the stopping point of your swing., just remember... hand in motion, pull trigger, spray, release trigger, stop hand motion.
Should you mess up and load an area with too much paint simply take a brush and spread it out.

Maximum transfer of paint is accomplished by holding the gun at the proper distance from the surface to be painted. Too close and you apply too much paint, too far away and you introduce a lot of spray into the air as it is not hitting the surface. This distance varies and depends on the amount of pressure you are using and the viscosity of the paint so experiment until you find the proper distance, normally 12" or so is about right.

Cut pieces of cardboard about 12" wide an 3' long and use them as a shield holding them up against the areas you want to protect. Cut several and when they become loaded with paint lay them aside to dry and you can reuse them later. Its a good idea to keep a bucket of clean water and a sponge handy too, you are going to slip up and overspray onto something you don't want painted and this way you can clean it easily. When you have finished be sure and clean the equipment up, most rental places charge a pretty hefty cleaning charge if you return the unit dirty.

Put the dip tube into a clean bucket of water, back off on the pressure and put the gun into the 5 gallon pail of paint holding the tip just under the surface of the paint. The line has about 1 qt of paint in it and you can push it through with water. As soon as you see the water coming out release the trigger, move the gun over to the bucket of water again holding the tip just under the surface, and pump the water for about 2-3 minutes. Dump the water repeat the process until the water runs clean. Normally 3-4 rinses are necessary. Power OFF! Trigger the gun to release the pressure , clean off the exterior of the gun with a wire brush, remove the tip, clean it well and replace it.

Be sure and replace the filters before returning the unit if you removed them earlier.

Next time you paint why not Insulate!
Hy-Tech Insulating Ceramic Additive for Paint!