UPNP is pretty important for a lot of online games.
Typically, it can be possible to join another lobby, but impossible to host (insofar as other people can't connect to it)
Of course they can be opened manually but that assumes some technical experience, and that the ISP provided hardware gives you access to its configuration.
Normally people say games. I have disabled upnp on my firewall and there're two gaming PCs, a PS3, a PS4 and a PS5 running happily behind it. I just finished a Demon Soul's session with voice chat with friends with no problems. NAT type 2, because I managed my firewall to enable this.
ETA: My point was to get an example of something that breaks "because it doesn't work without upnp". I have yet to see a game that doesn't support a fixed set of ports.