Medium's pitch was that they would drive traffic from other sites/publications/etc. in their network to yours. So by being in the network, you would get a free

Medium's pitch was that they would drive traffic from other sites/publications/etc. in their network to yours. So by being in the network, you would get a free promotional boost you wouldn't get by setting up a standalone web site. (Of course it eventually turned out that the size of this boost was not as big as many anticipated it would be, which dampened the appeal of being in the network somewhat.)