Photo ops.
For example. I want to see Elijah Wood, I have his auto a dozen times over, I don't really want it again, but if the showmasters go down their usual route of stopping photos with the guests, then photo ops are a brilliant opportunity.
To try and get a photo with Elijah is the only reason I am going, to be stopped on reaching guest, after paying for another auto I don't want is so damn annoying, so I will be happy to pay for a photo op, at least it is guaranteed!
And it has always worked well at American cons.