Welp, I just researched it a little bit and I gave it a shot.
It streamlines creating mods for almost any valve game. I've setup my own test mod on top of MapBase in under 10 minutes. I doubt I could've done that without wasting a good hour on reading how to do this (I haven't been a regular mapper for years and wasn't really into modding so I've forgot a lot of details).
Here's proof:
You can easily set background images (I've AI generated one for this) or map-based backgrounds.
This is how the main menu of the tool looks like:
In the free version you can only access the mod assistance tab and the mapping assistance (which you could live without). So really, it's mostly just for the modding assistance.
Unfortunately, it is a bit buggy (some menus don't load unless you restart the app), and I despise the payment model. I'm not saying the author doesn't deserve to be paid for the work put into the tool - it's clearly a great modding assistance tool that Valve should've provided - but it's such a niche product that I don't think it deserves a subscription model...
All in all - it's a really great tool set back only by its payment model (imho). The author asks about 20$/year, but I think it should be a one time price.