Weather conditions can be unpredictable at times, and a wet camera is the last thing you want. I keep a one gallon Ziploc bag in my camera bag at all times. My camera bag is water resistant and can take a bit of rain, but it makes me really nervous if I know my camera could be getting wet. My camera and lens will fit inside of the plastic bag, and it serves as an extra layer of security against bad weather.
A Ziploc bag is also a good idea for extreme temperature conditions. If you wear glasses, you know that they’ll get all fogged up when you go from winter weather into a nice warm house. Your camera will do the same, and condensation can lead to damage. Throw the camera in the baggie before you come in, and help ward off some of that moisture.
A one gallon bag doesn’t take up much room in your camera bag, and you never know when you might have a use for it.