One advantage for adding a brass part somewhere in the mix is it is a good compromise between hard enough to do the job well, yet soft enough to ensure that the other mating part/s will NEVER wear. ie: the brass part can be "sacrificial" If it is also significantly less pricey than the mating surfaces, this is a good choice.
I have no first hand experience with this specifically with triggers, but in the machinery I work with it is a constant. Also for 1911 pistols, i prefer Brass grip bushings for this reason.