It is called an indusium and helps disperse the fetid smell of the stinkhorn's slime over a larger area. The slime attracts the flies which carry the spores around.
Amazingly the stinkhorn drops its net in a matter of 30 minutes, which means you can literally watch this happen in front of you in real time if you really wanted to.
Someone should have thought to do a time lapse like this of that Phillipino Swamp Thing guy. That's what his makes me think of: horrible, mutant strains of HPV.