From only reading the site, my guess is that the product allows you to select a rectangle, and whatever apps' window surface is in that layer stays there. So I could highlight the bottom 40px of my slack window and those bottom 40px would always be visible and updating even if I open another window over the "snap".

That's how I'm reading it. If that's actually true, that's pretty compelling. A "bring to front" of any portion of any app that keeps it live.

If not, then either I'm way off base or the site isn't clarifying the purpose behind keeping a snap "floating" - why is that a marketable point?

Edit: typo and clarification on final question