Well I've always seen it as intimidation or a ritual. We're all very aware of how ritualistic Predators can be, And although we do not know what they have in mind when they do it, We can be safe to assume that it is just part of there belief system.However, it could just be that they want to scare the living shit out of its preys friends. Which i also think is a strong possibility. It sure as hell sends a strong message.
We can also consider that the Predators drain the blood to allow for ease of trophy taking, But you'd have to ask SR about that one.
Edit: I looked back on some old threads and found this comment by Stalker
The predator, to me anyway, only sems to make trophies out of prey which put up an honourable fight & which have the guts to stand up to it, 1 on 1. There's hardly any honour in taking a trophy of prey which you just shot while they were unaware. As for the skinning & hanging, the preds use that as an indimidation tactic, to strike fear into the heart of their prey, it makes the hunt more interesting, it'd put the $#its up anything to see their own kind skinned & hanging upside down. It's is also used to show their hunting territory, to ward off rival hunters, letting them know that this is their preferred hunting ground.