The more you can see through an element, the more its tangibility decreases. Pairing “barely-there” buttons, for example, with full opacity ones in a UI design can easily communicate the difference between on/off or selected/deselected states for a feature.
by Oleg Frolov in Interaction design
With the rise of neuomorphic design trends to create clean, simple appearances in UI designs, adjusting transparency is a simple—and minimalist—way to communicate transitions.
by Vadim Demenko