Here's a simple example of a use case for this. Imagine you're laying out a poster for elementary school science class. You have some graphics and text in various places and you want to put a periodic table of elements near the lower right of the page. Periodic table is, basically just three rectangles subdivided into squares. So in Illustrator, I'd draw three rectangles, then Split Into Grid and then go about coloring in each one and giving it a name, using duplicate/repeat (i.e. Smart Duplicate in Designer) to move text labels.
Using Designer's Snapping Grid feature, there's no way to drag the grid around to control where it starts. Using Guides means getting your calculator out.
More artistically, use it to create a grid of large pixel-like squares that you color in separately.
It's such a common thing to have a rectangle of arbitrary size and need to divide it up into rows and columns evenly with controllable gutters and so on. Snapping Grids, being fixed to an origin, just don't help (plus you'd then have to go create each rectangle in the grid by hand which is annoying even with Smart Duplicate.)