Imagine you need access from a remote system to a local system, that is allowed to open ssh sessions to the remote system, but the remote system is not allowed to access your local services. This can be solved by using ssh tunnels.

To enable access to a service on port 389 on a local system, open an ssh tunnel to the remote system source with

ssh -R 10389:localhost:389 -f -N source

to system source. Now, you can access the local service on localhost:10389 on the remote system.