I don't have access to an exploded view of the stick but generally there are several reasons for a sleeve. If it is under the CP on the bottom of the stick below the pivot point, it can server the purpose of increasing the working diameter of the stick for better switch contact, insulate the metal stick from touching the metal switches, and as a spacer to better compress the spring (if present) on the stick. If it is on the top of the stick above the control panel it can be used as a spacer to keep the stick from falling down through the grommet, increase the diameter of the stick to limit movement, protect the hole in the CP from damage/noise from the metal shaft.