Why is there an allen wrench socket in the top of it?

 

When you're tightening the at the upper spring perch, the shock rod will turn with it if you don't use an allen key to keep it from spinning.

 

Why does it spin? are they adjustable? I figure there has to be a reason that they spin.

 

the rod is a piston that just sits there so it spins. not adjustable. all shock rods spin.

 

yeah when your taking off/apart (loosening) the shock/spring assembly or putting it together (tightening) you use an allen key/hex key to hold the shock rod stationary while your use a combination wrench to tighten the top shock cap/top nut.. otherwise as you turn then nut/combination wrench.. the shock rod keeps spinning and it won't break loose or fully tighten since the shock rod keeps spinning...