Clearance requests must be accompanied by a justification, which basically means "needed to work on Project XYZ". You cannot get a clearance unless you actually need it to do work, and should not be cleared any higher than is necessary[1]. This leads to the chicken-and-egg situation with cleared positions: you need a clearance to work on these programs, but you can't get a clearance for the first time until asked to work on one of these programs.

[1] Some companies will try to put you through for higher clearances than you actually need right now in anticipation of future work, but they aren't supposed to.