The spacer is for use with the older bikes that have shorter push-rods.
It's important to note that Ducati slave cylinders are not interchangeable between the pre-2001 and 2001+ superbikes because the push-rod length is different. When Ducati solved the slave cylinder leakage problem in 2001, they modified the slave cylinder on all bikes, except the SPS. That modification to the housing design required the use of a 10mm longer push-rod.
Consequently, some of the slave cylinders on the market come supplied with a 10mm spacer so they can be used with either pushrod length.