Why do we need to install ubuntu when we already enabled WSL?
It depends on your needs:
Ubuntu available in WSL is somewhat dated, it is not the most recent version of Ubuntu. You can, however, boot docker images and pull more recent versions or other versions of Ubuntu or other versions of Linux.
WSL is not running on a Linux kernel, so some Ubuntu / Linux features are not available. Graphical apps are mixed and although you can run an X server on Windows, I do not believe you can run Wayland.
In addition to kernel apps, not all programs available in the Ubuntu repositories will run in WSL.
So if WSL does everything you need, and you are not developing say Wayland apps, it may work fine.
On the other hand, there are some features that some users and / or developers need that are not available in WSL, and in that case you would have to install Ubuntu / Linux.
It is difficult to impossible to list everyones needs or all the limitations of WSL here, in this answer, but for additional information see :
See What are the limitations of the Windows Subsystem for Linux? for some limitations.
See also https://docs.microsoft.com/en-us/windows/wsl/faq for an overview of WSL limitations.
See https://github.com/Microsoft/WSL/issues/ for ongoing bugs and missing features.