Sheepdog, the Benelli button is much easier to explain in person where there can be demonstrations rather than just in text, but I will see what I can do.
The button is actually called the carrier release button, and it is located on the front-right side of the trigger guard.
When the chamber is closed, pressing the button will release a shell onto the lifter. For most people, this isn't a help or a hindrance when compared to other shotguns. It could be useful if you don't want to keep your defensive gun with a round in the chamber. You keep the bolt closed, fill up the tube, and then press the button. Then, when you need to get the gun running you simply have to work the action and you are good to go. Of course, you could ghost load as well to get that extra shell in the gun, but that's another story.
A 2nd use for the button is if you want to do a slug changeover. If you want to remove the shell that is currently in the chamber without a new one loading from the tube, it is easy, just don't press the button. The gun will not release a shell from the tube no matter how hard you rack the bolt unless the trigger is pulled or the carrier release button is depressed. To perform a slug changeover, you can use your weak hand to rack the bolt back and hold it to the rear while dropping the slug in the receiver (with the same hand). Any rounds that were already in the tube will remain there and you will now have a slug chambered.