There are frequent posts here advising people to have a moisture trap between their compressor and airbrush. I've always had one and never get moisture out of my brush or tools. Today I was bleeding my compressor down and noticed the amount of water in the trap. I used the compressor for about an hour, and during that time I remember hearing it come on twice. It has a 5 gallon tank and compresses to 140 psi and then runs down to around 120 when the motor comes back on. I don't know how to calculate the compressibility of air, so I'm not sure how many cubic feet that actually is, but it isn't a whole lot.
I pulled the moisture trap off and measured the water in it and there were 4 cc of water. That's only a little over 1/8 ounce of water but if you don't think that's quite a bit, fill a 4cc syringe and see how much it is. Then imagine that water coming out of your airbrush while you are painting. Probably not as big a deal if you spray acrylics since they are water soluble, might make it take longer to dry, but for those who spray enamel or laquer it is a big deal.
Bottom line, for less than $20 a moisture trap is very good insurance against a trashed paint job.