From CreativePro.Com Graphics –Add Gradients to Strokes in Illustrator CS6:
Strokes are one of the essential building blocks of graphics created in Adobe Illustrator. Creating certain effects, however, required building stroke upon stroke upon stroke, thus resulting in complex files.
Now a new feature has been added to Illustrator CS6 that allows you to add color gradients to strokes. This capability lets you create sophisticated effects within a single stroke.