Well there's some science I've read about the tunica sheathes. You got 2. One lateral and one that extends length wise up and down your shaft (girth, length respectively). When you stress one, lets say lateral (from jelqs or whatever) the girth expands and stresses this part of the sheath. HOWEVER, the tunica sheaths are like a spider's web (but more complex), both cross-sectioned to each other.
So, when you stress the girthy sheath and the lengthy sheath at the same time, you're achieving maximum allowance for expansion in both length and girth. Basically by weakening both sheaths in the tunica you can gain more efficiently.
Lets say you only worked length, you'd really only be stressing the lengthy part of your tunica sheath, and so the under-expanded lateral sheath (due to refraining from girth work) isn't even being stressed really significantly. Logically, if you weaken both portions of the sheath every day, you're more likely to gain because there's less overall resistance. Basically, girth and length should be worked together in the same day, and probably length first, IMO.