Why do I need a 9000lb winch if my trucks weight is 5000lb and most trucks out there on trail do not weigh 9000lb? Another question,does a snatch block cut the weight in half?

You need a winch with a higher rating than your vehicle weight because it will have to pull not only the weight of your vehicle, but also have to overcome whatever is holding it back. Wether rocks, mud, or gravity, it is going to make the pull harder. Think of it this way, you can take off from a stop in high range easily, now put the front tires up against a big ole rock and try it in high range, probably stalled out, but the vehicle doesnt weigh any more than it did.

A snatch block does not change the weight of the vehicle, its impossible, what it does do is add more mechanical advantage to the winch. Same example, high range up against a rock, stalled out, use low range (more mechanical advantage through gearing, just like a snatch block) and you crawl right over the rock.

Give a man a big enough lever and he'll move the world.

Thank's, I thought that is why, but needed to know for sure.

I winch myself about one out of four times I use the winch. The other guy's rig is always:
1. Bigger than yours
2. And stucker than you'll ever get.