Tools i use daily don't get anything in them before i use them, they get lubed by the automatic oiler.
Some tools like needle scalars can sit a long time and the small amount of residual oil may not be enough to protect them well for storage. When you put a few drops in the fitting it doesn't go into the air motor until the trigger is pulled or paddle depressed.
On these tools i hold the trigger while putting oil in (about an ounce), it flushes any dirt or moisture from the air motor and leaves a good oil film.
Spray guns, tire fillers, blow guns are on another 3/4" hard line with no oiler but uses a primary moisture separator and Devillbiss desiccant dryer with hoses never contaminated with oil.