A captive portal is a login screen of a service that prevents other activity unless logged in.

My guess is that Firefox will not try to check for updates or other things if it notices that the detectportal-request was not successful (i.e. the user is not logged into the hotel-wifi or something like this yet).

It also then can prompt the user to go to the captive portal page and log in, without the other redirection methods of the captive portal breaking anything (e.g. if they spoof DNS responses to redirect you to the login page) or the user being confused why they can't use the web despite being in the network.