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as the suspension on the rear dips down the camber increases(you got that bit yeah?)
when the camber increases on the rear of sierra/saff, the toe angle changes due to the geometry of the back suspension, on a std height car it only affects tyre wear whilst cornering hard and whilst under hard acceleration(when it squats), now if you lower the rear the camber is increased constantly as is the toe angle so you need to correct it i.e with the shims!
they should also help with standing start launches as the should keep the rear wheels parrallel when launching by showing the tarmac more rubber!!and decrease the dreaded squirming of the back end


HTH
Pugo