My boy has practically no withers (cob!) so I use it to stop the saddle slipping and also to give him a little bit of cushioning, as he's had problems with his back in the past and I was advised to use one.
No, I have a Rambo air pad as its what the saddler reccomended as I am on my 5th saddle in a year (horse keeps changing shape since I bought him and started working him) until I can afford the 6th