It doesn't do a damn thing to make it better. Dumb people think it's cool or exotic and buy it merely because they're dumb.
Purple weed is based on genetics and should your plant have a predisposition it can be stressed, fed certain ferts and be kept in a colder climate which causes chlorophyll to not be produced and anthrocyanin (sp?), which is the purple pigment to be seen. Purple plants are also highly indicative of a defeciency and so many times your question should be what happened to make the plant worse.