Heres info on one thats made specifically for Gen 3 Golfs/Jettas:

http://www.midknightmotorsport.com/products/bpc.htm

Anything you can do to stiffen the chassis will make the car feel more solid and reduce creaks/squeaks. The center brace tries to emulate the cross bars welded under high performance cars (like Robster stated) and in roll cages without taking up any interior room. The stiffer your suspension setup the more important it is to reinforce the chassis.

Of course tie bars are popular just because most people have them.