I own a 95 Jetta GLX and an 89 GTI 16V
my little brother owns a 96 Jetta GL and co-owns the GTI with me (project car)
my other little brother drives a 96 A4 2.8L Quattro
Definitely looks like a venturi. It mixes air with the outflowing water. Unless you want bubbles all over your tank, you're probably better off leaving that part off.