It used to be that port 80 was a hard requirement for Pi-hole, though that it less relevant these days.
The reason being is that we used to by default (though, it is still possible with some reconfiguring) serve a block page as a 404 for all Pi-holed requests, by returning the IP address of the Pi-hole and the rest of the url (e.g http://pi-hole-ip/images/advert.jpg, which doesn't exist, so 404 is returned)
But since the rise of https, the block page has become less relevant. The plan for v6.0 is to completely overhaul the web UI of Pi-hole, and remove the lighttpd requirement (requests will be served from a http server contained within the pihole-FTL binary - on whatever port the user chooses (at least, that's the plan, I think!!))
Keep in mind that with the current setup, your changes to the lighttpd config file will be overwritten on an update or a repair of your Pi-hole install. It's not ideal, I know, but we're working towards making things smoother in that department