Ruler And Snap Function

It's kind of hard to explain, but it basically makes dragging work in a similar way to how using the cursor up/down keys work in the Inspector spin box - moving by specific "chunks" rather than in smooth increments. So for instance, if you select an element, click within one of the offset fields in the Inspector, and hit Up, the element moves exactly 0.5 sp. If you use the snap to grid function, then dragging will also be constrained to exactly 0.5 sp increments, except it also rounds to the nearest 0.5 sp, to aid with alignment. So, if the original offset was -2.17 sp, then hitting the Up key changes this to -2.76, then -3.17, etc, but dragging up will change it to -2.50, then -3, etc.

If I understand correctly, you don't need a snap function for this, you simply need to attach the text directly to the staff, rather than attach it to the frame then manually adjust it. That is, use Staff Text, not frame text. You can then set the text style however you like in terms of font etc, and press "X" to flip it below the staff. All will work perfectly with no need for manual adjustments and no need for a grid.

Attaching to the frame would be a way to get it at the same physical position on the page regardless of the position of the staff, and that works perfectly well for that purpose without a grid - indeed I can't see how a grid would make that work any better. If you want it the place relative to the staff, then attach it to the staff.