Not including misinformed discussions (ie outgoing vs incoming ports), the two most popular reasons on clearnet for opening incoming ports are for running servers and torrenting.
The paragraph which covers hidden services could be made more explicit:
Hosting Location Hidden Services over Tor is possible. One way to accomplish this is using Hidden Services. To oversimplify this, by configuration Tor it instructions the Tor network to open an incoming port at Tor relays for Tor onion hostname which gets forwarded to a port on the machine where Tor is running (called virtport). Therefore, no ports need to be opened on Whonix-Gateway itself. The users real public IP address remains hidden. That onion hostname is only reachable by other Tor clients.
Maybe a more newbie-friendly explanation:
The reason why it doesn't make sense for Whonix users to open ports to the internet (for torrenting, or otherwise) is that opening a port without an accompanying public IP address is like handing out invitations with only a house number and no street name. It's meaningless. Obviously, giving away your public IP address would defeat the whole point of using Whonix in the first place. Hidden Services allow others to connect to you without revealing your IP address and do not require any ports to be opened.