That bolt and washer are the secondary part that retains the stub shaft of the axle assembly. It, in concert with the thrust shim and clip stop the shaft from moving in and out too much while the suspension cycles. Some people (believe) you can either simply run this without the shim and clip or just simply run a shim and clip. It was designed to be ran together. Here's a basic diagram, somewhere on the web I have the full pics/etc. (ih8mud IIRC??) Make sure you got the proper thrust washer and clip. If they're not there, run 'em.