It’s basically an extra layer of polish to make Debian as user-friendly as can be, which while being very pleasant to work with for experienced users, is likely to be particularly appreciated by those who are not particularly technically inclined. As an example, the Mint Software Store is pretty much unmatched as a stable, and extremely user friendly way to manage and install software, with reviews, Flathub integration, screenshots, etc. Where as on standard Debian, the GUI package manager would likely be Aptitude, which is quite a daunting piece of software for the uninitiated.

You could make a vanilla Debian install as user friendly as Mint, but you’d already have to have the skill to get it to that state, where as Mint is ready out of the box.

1. Easier installation.
2. Mint configuration of desktop settings
3. Mint tools (Warpinator, Hypnotix)

In theory, faster updates compared to Debian Stable.

I haven’t compared the repos directly though so I’m not sure what the current differences are specifically.