why does the eclipse debugger use localhost for debugging ?

The JVM is basically started using a network port for debugging. This allows the debugger to be on a different machine, which can be very useful at times.

Now when you're debugging locally, the JVM has been started on your local machine - the localhost IP address just refers to that, the loopback adapter for your local machine. You can attach the Eclipse debugger to a JVM running on another machine using the "Remote Java Application" debug configuration, but most of the time you'd just debug locally.

You can ignore the "localhost:" part, basically - it's just telling you which port the JVM has exposed for debugging.