In some ways it doesn't...it will depend on setting and the type of horror/suspense, and the scene... a great way to up the tension on a scene where you're playing on a moment of anticipation/dread is to treat the ambient and incidental sound the same way you treat the visual background to the scene; close in on the protagonist, shallow depth of field- and at the same time fade down the ambient sound a bit and bring up the protagonist, so you get more sound of their breath, any small noises they make... be careful to do it gently, and don't over tip it or you'll sound like an early 80's straight-to-vhs flick (sometimes not a bad thing... Video Dead remains a favourite of mine) ... at the same time, as Raguel says, the ambient sound is really key - think carefully about the key emotions, then use layers of sound to induce that, but be subtle unless you want to really hammer a moment home; want the audience to feel a little uncomfortable about the abandoned shack? throw in a low level sound of flies in the background... want them to throw up when the heroine opens the bathroom door and finds the 6 month old corpse of the cop? push those flies to fore (and maybe throw in a "my shoes are sticking to the floor" kinda noise)