Yeah, Inkscape Preferences doesn't have a Save or Apply button. Whatever you change there, it immediately is changed. Except there are a few settings which will tell you that you must restart Inkscape, before it takes effect.
I have not used those options for grids in Inkscape Preferences. And I'm not sure how they are different from Document Properties > Grids tab. I guess it must make the change globally, to all files, from Inkscape Preferences; while Document Properties changes it only for that file.
Also, you have to enable grids, before you can see them. Try View menu > Page Grid. And if you use Document Properies, there are some controls there for whether the grids are displayed or not. (And there's a key shortcut as well.)
And those apply to the grids which are used in Inkscape as helpers or guides. They aren't actually part of the image. If you want grids which are made of nodes and paths, you can either draw them yourself, or else there are some extensions which will draw grids.