In theory a tilting loupe is intended so that you can look in the direction of the light (ie towards lens aperture) when you look into edges of the ground glass for a wide angle lens, this makes the image brigher and you can more easily focus there (or generally adjust tilt, which brings the loupe to the edges).
I do not find it so useful in practice though. Tilting loupes are usually only 6x or so which for me is too little for the precision in focusing we require in digital era. Also with a good bright ground glass, which I strongly recommend if you're working with wides, tilting is only necessary on the widest lenses, and with those the need to look with the loupe in the edges is limited, since 1) wide angle lenses are tilted less often and 2) when tilting is done setting tilt from a table is usually effective, because it's a wide angle.
I got the Silvestri 6x tilting loupe to try it, I did not like it. The quality of the tilting base was no good either. It was actually easier to use a loupe with detatched base and tilt and hold the distance with the hand, resting it on the ground glass surface. I use the 10x silvestri this way in the very rare occassions I tilt a loupe. I can't remember last time I did it though.
With the tiny 20x I'm using (while waiting for the forever backordered Silvestri 12x) tilting is not possible since with this magnification you are so close to the glass there is no space to tilt. If I need to look into an edge for focusing I simply focus first with the camera pointing right at the target, and then compose so I don't need to look into the edge when focusing. If it's for fine-tuning tilt on a wide (eg SK35XL) I have yet not shot in a situation when a table + experience is not adequate. With a 90mm lens looking into the edge for tilt refinement makes sense, but then brightness is no problem so you just look without tilting the loupe.
