I use them to give me harder/softer suspension in corner without having to compromise on my springs as the are only active in the corner and not on the straight. For myself they are a valuable tuning aid. Say I was needing harder spring on the front but was happy with my suspension/ handling on the straight then I could try a harder sway bar.
This is just my opinion but it works for me. Others may have a better exsplination though but you get the point.
Oh forgot usually yes the harder they are the thicker or in xray case they have markings also