The purpose of using buttons with only an icon is to provide an easily accessible way to trigger a common and distinct in-app flow. For example, in
Material Design, a
Floating Action Button (FAB) is used to quickly initiate a primary action on a page.
FABs are typically a circle or square that is placed in a prominent location to make it stand out on the page. When clicked, it will typically trigger a frequent to use action such as creating a new item or initiating a new process, user flow, etc.