Question about proxy lists

Sorry if this is dumb, but I'm new here. Also sorry if this has been asked before (Yes I did search).

First of all, I must say that I was surprised to go to the "Proxy Lists" section and see posts containing huge lists of IPs and Port Numbers. Even though it was in the title.

My question is, where do these proxies come from? Whose IPs are these? Also, what benefit do the people posting these lists get from posting them (ie, why not just keep them to themselves)? And why are the IPs in the lists willing to let random people use them as a proxy for free?

Yes it's really cool and all that these lists are here, but it's surprising that there are so many free anonymous proxies out there.

Thanks.

Two main sources:
1) Scraped from websites that list open proxies.
2) Scanned with whatever proxy scanning software you like (I like nmap myself).

Free because:
1) Misconfigured routers and network appliances.
2) Corporate proxies that are misconfigured to not have access restrictions.
3) Deliberately set up to capture network info.
4) Honeypots to catch spammers.
5) Some people might just set up an open proxy for other people to use eg. to get around the Chinese firewall.

Bear in mind that only about 10% of the proxies you see listed anywhere will actually work for you and be anonymous - you always have to test them before you use them.