Why would you need this? Well if there are any applications your trying to run on your WSL that use graphics you will need Xming installed and running. Once you have Xming installed and you’re in your WSL environment you will want to set your $DISPLAY variable to the following.
The reason you add the localhost part and not just the :0 is due to the fact that if you remote into another system and you want to be able to have that system send something graphical you will need this. Please note that when you remote into another system you will need to connect to the system with the –X to trust X data sent back through your connection.