If you are seeing them on Jeeps and Toyotas most likely in the Jeeps its just a long link arm which is part of their 4/5 link rear suspension setup, not ladder bars. Google extreme duty 6" Rubicon Extpress Jeep Tj and you'll see what I mean.
They are usually used to limit wheel hop and keep axle wrap under control.
I'm not a big fan of them. Though I do realize that they must be used in many situations, proper leaf spring build up can eliminate most of the original problem of axle wrap. Like I said though sometimes you can't avoid it (35's, doubler, torquey diesel motor ext ect.)