Why Do I Keep a Copy of These Files?
TL;DR: We need copies in case these files are deleted or if the file hosting service shuts down.
I started saving Android content after CyanogenMod's shutdown in December 2017. Everything happened so quickly that there was no time to make a complete copy of the website, wiki and and builds. I was lucky because someone managed to get a copy of the last build for my phone and uploaded it to the Internet Archive. In any case, even active projects like LineageOS end up deleting old builds, that's why I created the "android content mirror" page where you can find content for some Android phones.
I keep copies of custom ROMs, GApps, firmware, apps, and other content that I think it's useful for popular devices/devices that I own. Some of it is kept because it's hosted on free services that can be closed anytime or aren't available for everyone. I like AFH but I don't know if it's going to be online a year from now. Google Drive seems to be a popular service, but it's not available in some countries. Then we have some websites that require a login to download the content (eg: 4PDA... and they use cyrillic captchas).
Where's the content hosted?
Most content is hosted by me, but some files are hosted by the Internet Archive (IA).
Ideally I would host everything, but my intention is to have these files online for as long as possible and hosting everything is expensive, so I decided to use the IA for heavy content (custom ROMs, for example).
Are you getting something out of this?
No. I do this for myself - so I can have a copy of important stuff - and also to help other people. I enjoy doing it. That's it.
You can see how much it costs to run this website on the donation page.