Sleeve in seatpost clamp neccesary

[eeksll]

what does this sleeve do? do I need it? I put a old quick release seat clamp on another bike and this piece is missing, can I put something else there if its required?

[Duck!]

That's your clamping surface! Without that steel shim, you'll have two comparitively soft aluminium surfaces rubbing against each other, which will wear the clamp out much faster.

[eeksll]

just as well, the seat clamp isn't clamping anyway