L2 gateway

I am trying to understand the L2-GW concept in openstack neutron. I understand that its primary purpose is to bridge two l2 networks into one single network. We have tenant and provider networks in neutron. 1. Why would anyone want to bridge two l2 networks in the first place ? 2. Can these two l2 networks be, two unroutable tenant networks ? Please bear with my ignorance here. I am trying to grasp and learn these concepts. Any help in this regard is much appreciated... Thanks elinux