Spindle Extenders are used to lengthen the distance between the object being held and the headstock. 3" of extension is typical.
The purpose for such an extension is to enable clearance for tools to get to the object.
Disclaimer: eMail comments to me at OTBookOfKnowledge @ Gmail.com. The process of woodturning involves the use of tools, machinery and materials which could cause injury or be a health hazard unless proper precautions are taken, including the wearing of appropriate protective equipment.