How can I find out what a switch does?
Like someone said on the comments, sometimes a house is wired so that there are two lines going to a fan/ceiling enclosure: one for controlling the fan, and the other for controlling the fan's light (tho some fans' wiring don't allow for this type of setup). So one switch could be just going to a ceiling enclosure (where there might be just a light) and it's not connected to anything.

Edit: just saw your comment that's unlikely for a fan. Another possibility it's that it controls one of the power outlets.. you could test this by plugging a small light/hairdryer/w.e. and switch the light to see if it turns on/off the appliance you connected.