Yes they are adjustable for the reasonings Jeepn stated above. When you put taller springs in a TJ with the stock control arms the angle they sit at changes drastically. This affects many things from ride comfort to wheel base. So the adjustable control arms help you bring back some form of the stock suspension geometry.
As far as flex like jeepn was talking about the aftermarket arms (except Teraflex
) mostly use a sperical ball joint like a Johnny Joint or a Heim style set over the stock rubber bushing type joint. This allows for a greater degree of rotation at the axle end hence the added flex.