Do as sirsparksalot suggested and check out the "unused" switch. If there are no wires attached, it simply is being used as a filler -- the person who put the switches in couldn't find a four gang box, and used a five gang box instead. You can buy a blank switch plug which would fill the space without it looking like a switch, but why bother.
On the other hand, if it does have wires hooked to it, it may or may not do anything. I have seen switches which turn on lights at the other end of the house, the garage, or maybe an outdoor circuit someplace. Unless you can trace the wires, you may never find out where it goes. I have seen extra switches wired to a basement or attic so that a future switched function could later be added without having to run more wires through the wall. I have seen times when the switch was made inoperable when a half-switched outlet was replaced without removing the side tab on the outlet. This kept the entire outlet on all the time, and kept the switch from working. It may also have been disconnected for some reason -- like the switched function was no longer needed or desireable. At one house I inspected, there was an extra switch that didn't seem to do anything, but it turned out to be on the doorbell transformer circuit. I probably would never have figured that one out without a conversation with the previous owner. The owner was a day sleeper, and would turn the doorbell off before going to bed, but would turn it on when expecting company or at times when they were awake. I have also seen garage door openers wired to an inside switch to keep them from being remotely opened when the owner didn't want them to be operatable, and one house had some type of heating mat installed in the poured concrete steps to melt snow and ice. A switch near the door controlled the circuit, but unless it was snowing, you probably would have never known.
A good electrician or electrical contractor will have access to a wireless probe which can be hooked up to wires at the switch and used to follow them throughout the house. The really good ones can sense wiring throuhg many feet of ground when used outside. These are used by contractors to locate buried, hidden, or broken wires in construction and demollition jobs.
To conduct a do-it-yourself test for "hidden" usage, turn the swtich "on" or flip it up if not marked and leave it in that position for a while. Put tape on it if you think you or someone else might change it. If anything electrical in the house, garage, or yard doesn't work, throw the switch to the other position to see if that corrects the problem. If not, switch it back to where it was. After a few weeks, and trying everything you can think of, throw the switch the other way, and run the same test. You will need to plug a lamp or AC radio into every outlet you find of course since many people have outlets they rarely or never use.