A battery switch allows you to disconnect the battery from any and all loads. There are many phantom electrical devices that just use a little bit of juice..... even if disconnected from shore power. If you do not have shore power or solar when you are stored the battery will simply drain itself of all power because of the little phantom loads.
The battery switch allows you to not have to worry about that. Then when you are ready to start things up after winter storage you will still have power in your battery.