Its a safety feature in case the spring breaks. It will almost always fail at the loop. The tab on the rear is to limit the spring travel to help prevent it from breaking.
I just addressed a similar issue in a thread in this section about swapping in 1 ton springs.
They were dealer installed. It was a recall issue and this is how GM "fixed it". I cut all mine off since I changed springs/hangers. They areant needed with double (military) wrap springs.
The thinking was, that if the driver side spring broke while driving on the highway, it would cause loss of control and most likely shoot you into the other lane. If the passenger side broke..well you still lose control but would shoot off into no mans land away from oncoming traffic
You shouldn't need them with aftermarket springs. The new springs should have the double wrap.