They allow the adjustment of Caster and Camber settings in the front. I'd say since your Mustang has been lowered it was required to install them to get the geometry right up front.
you get the adjustment as mentioned above. plus, they're stronger than stock (especially if you have a 4-bolt version), and the mushy stock rubber bushing gets replaced with something much better (like a spherical bushing in the case of my MM plates).