Large and close buttons can be clicked easier
Paul Fitts developed the law that buttons need to be large enough so that they can be hit easily and actions that might or will follow each other sould be placed close together.
This law is important to UX designers as they want to keep the usability as high as possible which can be done by creating large buttons for important interactions with the product, software or service. An online shop for example is made to buy products or software online. Sometimes users want to buy one product multiple times so the quantity field or dropdown should be placed next to the buy button. If an online shop sell clothes the customer needs to choose a size. This should be done closely to the buy button, too.