Lower Bay subway station is a very important part of the Toronto transit system. It's primary function is to route regular subways, called "Revenue Trains" because they generate revenue with paying passengers, and subway workcars which are the (typically yellow) trains used to transport work crews, supplies, and equipment to jobs throughout the system (usually) during the overnight hours. It allows for easy access from the Yonge/University/Spadina line to the Bloor/Danforth line and vice versa. For those of us that worked here, we called it the YUS (north/south) line and the BD line (east/west) for short.
On occasion, the Lower Bay Station is used for filming movies or commercials which is why you may notice American advertisements on the wall as a reminder of that.
The station is also used for storage and can be dirtier than the public stations.
There are reports of a ghost haunting this station which you can learn more about on the main page of this website. (Have I ever seen it? No, but that doesn't mean anything to those that have seen it.)
