There are many different uses and scenarios for tunneling traffic on the internet, but in regards to shell accounts maybe the most common is SSH tunneling. Using SSH tunneling to reach destinations on the internet without revealing your true source IP can be used as an anonymization technique, and is not allowed on Blinkenshell. For that use-case we recommend getting a anonymous VPN service.
You can also use tunneling if you for example have a home server/rpi/NAS running a HTTP/TCP/UDP service that you want to publish on the internet. This is allowed on Supporter accounts, and you could use a program like https://github.com/fatedier/frp to accomplish this.