Depends on the horse and what my saddler recommends. For close contact saddles that can't be adjusted they are brilliant but they are too often used as a replacement to a saddler which I don't think is a good thing.
I've got one as recommended by my saddler; it's there while we build up her topline, although I will probably buy her a replacement saddle when I find some cash as I think we could possibly find a better fit all round.
I'm currently using one to optomise saddle fit on a 3yo. He's hacking out lightly, and the saddle he's in will probably fit him quite well once he's filled out next year. In the meanwhile the dead sheep pads him out a bit so that the saddle sits as it should.
If your saddle is a little too wide for your horse, then a half pad can be very useful. I prefer the ones with pockets for removable shims, so that you can customise the fit. Very handy during fittening work etc. as the horse changes shape, though of course the saddle must be suitable for the horse, as a pad cannot work miracles.
If your saddle is a little narrow for your horse, then no .
I have one, with removable/adjustable shims, for a horse with severe muscle wastage and it has made him very comfortable as he rebuilds the muscle behind his wither and under the back of the saddle area. Works for him.
Depends on the horse and the saddle fit. I like them on my horse as he has a wide spine but a high wither so gives him more comfort and support. Also like them for jumping as takes concussion impact. People say you wouldn't wear thick socks with fitted shoes on. But if I was carrying a heavy ruck sack I think I'd like some padding on my back!