It actually had, or had, another more important function. You would use it as a stop to limit how far the wedge could be driven in, adjusting as needed to maintain proper barrel/cylinder gap. But this function was abandoned soon enough, and it is mostly used to help retain the wedge. By the 1870s that would largely be it's only function, as witness the elimination of the spring for the simple short cut slot on the wedges of the factory built conversions.