What is involved in a WSL "distribution"?

WSL allows the user to use any distribution of their choice, and Ubuntu is installed by default. I don't really understand the relevance of the distribution in the WSL context. My understanding is that Linux distribution refers to the skin of the OS. A UI layer on top of the OS core. But when using WSL you just use the command line, or perhaps run an independant single gui application. So what is the relevance of distribution in this context, and what difference does it make which distribution you choose?