Most mice use a scroll wheel design to define the third button, allowing users to scroll the page. It can also be used as a shortcut key, allowing people with poor eyesight to zoom in on anything on the screen at will. For example, in the Chrome browser, you can hold down the Ctrl key and then scroll the wheel to zoom in and out of the page.
There are also some game mice with custom keys such as side buttons. You can download the driver from the official mouse website to define the functions of the side buttons.
