Cages are for attaching accessories. I'm not a fan of cages but the A7s has absolutely no where to attach any accessories. The hotshoe is of a smaller size than the standard one and has very fine, fragile looking electronic contacts.
It is what sony calls their MIS (Multipurpose Interface Shoe) and contains connections to a host of audio accessories including their XLR adapter. I keep it covered up with a plastic protector and will not risk wearing down the contact by inserting accessory coldshoes into it. Problem is: the XLR adapter itself needs another coldshoe to stand on. Hence, some rigging is needed. A cage is one solution to this problem.