Once there's a large group of "needful" collectors, the other collections (the kind that want to make money) show up. If an item becomes "hot" with needful collectors, the speculators will buy it up, creating scarcity and driving prices even higher.
This works out pretty good, until a disruption happens in the market. Sometimes the market gets flooded with low-value merch (how many X-titles were there?) Sometimes a couple of movie bombs ruin the property (exhibit number 1: "The Fantastic Four.")
You see this disruption in the "real world" too. When was the last time you saw a "used CD store?"
But for the "needful" collectors, the value is in having the thing. And the more the thing costs, the more value people place in having it.
As a previous Pez collector back in the 7th grade, or "Pezhead" (back in the 90s using my paper route money) who yes, attended several Pez conventions at the time, I can attest that the draw was in-part connecting with other people around a shared interest -- the community.
Although some of the email on the big Pez email list at the time [1] was buying and selling, a lot of it was joking around, organizing meet-ups, sharing stories about a great "find" at a garage sale etc...
For me, the ability to connect via email with people from around the globe talking about a subject like Pez was what stuck with me. Although my Pez collecting ended, that unifying aspect of the internet continued to inspire me through my formative years and continues to this day.
That said, here is an indie documentary about "Pezheads" if you are interested in hearing more about crazy collectors: http://pezheadsthemovie.com/
[1] http://www.pezlist.com/ The Original Pezheads Email List, online since 1995