The most common use of buttons is to secure the openings of garments, by sliding the button through a slit in the piece of cloth to which the first is to be held. The slit is only sufficiently wide to allow the button to be fully pushed through while still remaining stitched to the first piece of cloth.
Buttons are also be used for decorative effect, with or without a practical function.
Buttons are found on all sorts of electronic & mechanical devices. You could push a button in order to summon an elevator or to turn a stereo on or off.