Hey,
I know they make left and right handed jumars, just curious which people use and why?

They make them for both hands because you need a pair for jumaring fixed line while aid climbing. If you are taking about the fixed line on Denali, for example, then you only need one and you need the left. You ascend to the right of the fixed lines, so will be using your left to manage the ascender.

ExcitableBoy is right. Jumars are made to ascend a vertical rope, using a pair of them. They are a type of ascender.

If you are talking about using them on a fixed line as a self-belay on a less than vertical slope, then they can work for that, but that is not their primary purpose.