Halfpads have heaps of uses depending on the pad itself - some are raisers - some are meant to release pressure and others make the saddle sit flatter on the horses back
Ive been told by my saddle fitter that if your saddle fits you shouldnt have to use a half pad though = In fact it effects the fit of a correcly fitted saddle and has been known to do more harm than good - i have heard of those gel pad ones actually hurting their backs
But i really think it all depends on the thickness, currently saddle fit discipline etc... if your wanting to use one ask your saddle fitter their thoughts....
In saying that in NZ you see alot of them in the show ring without a saddle pad for looks (In fact I use one for show hunter - jumping) or combined with a normal pad - for appearence
I completely agree with jody. When I was looking for one I was told that too much padding is not all that good (even though you'd think it should give more protection if the rider is unbalanced). I'm talking about just standard ones, not those to relieve pressure/clear withers and such. I personally prefer Thinline - it's indeed thin but still add some protection.