On a technical level Ubuntu 18.04 is based on a mix of Debian stable and testing with Canonical's additions.
Pengwin is primarily Debian testing, with some stable and some unstable here and there.
Pengwin configures dozens of settings for WSL and has optional WSL-specific features.
Settings are delivered by pengwin-base and features can be configured with pengwin-setup.
You could probably spend hours implementing these features on your own each time you have to install Ubuntu on a new Windows device.
But by purching Pengwin you support open source indie devs that handle it for you, answer bug reports, constantly add new featres, and are available for support.