SIDs (and STARs) are mainly used at bigger airports. If a normal departure is to go direct to "A" then "B" then "C", crossing "C" at 3000 feet and then going direct to "D" and crossing "D" at 5000 feet and departure control will be on 123.75, ATC can publish all that and call it the Apple1 departure. Then all they have to say is, "You are cleared the Apple1."