That was suggested over at VW Vortex in order to make the front seat feel more comfortable.

 

If you use a spacer make sure they are big, if you make them small the leverage can crack the floor panel, I usually use a piece of square tube cut in a long taper and put it front to rear so its held at both ends and cant lever the bolts.

 

That vortex thread contains a link to this forum and the thread started by the fellow who made the spacers.

The Fit's seat bottom pitch is poor for many full sized males. The spacers are reputed to help that.