Lots of answers about use-cases of additional wifi networks, so I won't go into that. I haven't seen the downsides mentioned here, though. While technically you can run lots of wifi networks of off the same wifi router/ap, each SSID takes a bit of air time to broadcast. While this might sound rather insignificant since this is only a rather tiny bit of information transmitted, it is actually more significant than one might expect. For one the SSIDs are broadcast quite often, but also they are always transmitted at the lowest possible speed (meaning they require a lot more airtime than normal WiFi traffic would require for the same amount of data) for compatibility reasons. This is also the reason why it is a good idea to disable older wifi standards if not needed by legacy clients (such as 54 Mbit/s 802.11G wifi).

Having two networks is usually fine and doesn't cause noticable performance degradation, having 4 or more networks is usually noticable, particularily in an already crowded area with lots of wifi networks.