Double pole circuit breaker and my wiring have me worried.?

I had a room in my house with an outlet on a switch. I hate those so I added a ceiling fan. The switch was originally an "end Line" switch. I rewired the outlet and the switch to use a "through" switch. The part I don't understand is the circuit breaker. It is a double pole breaker 110v. It uses 2 hot wires with a common neutral. One switch controls the light in the room I'm working on, the other controls the hallway. There is some sort of transformer on the side of the panel that HUMS. WHen I turned off the breaker to the room I'm working on, the HUM stops. I noticed that the Ceiling fan motor HUMS louder than I thought it would. Here are my questions. Why the double pole breaker? To save space? What is the transformer? Does anyone see a problem with my switching an "end line" switch into a "through" switch?? Thank you so much in advance.