I think the idea is that many wifi networks out there require registration/payment to use. They also do this in really, really weird ways because making stuff is hard.

Some of these only work by capturing http requests and rewriting them to take you to their portal. Funnily enough, that and other methods often work very badly and so you might be left trying to visit a site getting timeouts. Maybe your browser visited it previously and saw HSTS for example and so only tries https?

The point of this site is that when you realise this has happened, you type in 'http://neverssl.com' into your browser to force an http connection which hopefully the network will grab, mangle and take you to the login page.

It's a solution to a problem that really shouldn't exist.

Annoyingly, different client venders require different things. Here's an example of someone working on this: http://www.revk.uk/2016/08/captive-portals-apple.html (and a money quote from the comments: "OS X only does it if you're on Wifi though - for some reason they assume you'll never see a captive portal when wired"