First, that’s often not the case; the kashrus agencies rely one upon another and often share mashgichim. The cost of three hashgochos is probably a lot less than triple that of one.
The value is that the consumer knows (or at least assumes) the standards of each of the hashgochos is being upheld. So, for example, although the OU allows confectioner’s glaze, if the hashgocha on the Twizzlers sticker doesn’t, the consumer who is makpid knows he can eat Twizzlers. Otherwise he doesn’t know.