Web design. A transparent logo or icon drops onto any background colour or image without a white halo.
Social media. Stories, posts, and overlays use transparent PNGs to composite multiple elements together.
Ecommerce. Amazon and Etsy accept transparent images for some categories. For main images they want white, but for secondary images transparency is often allowed.
Presentation decks. A cutout of a product or a person drops into a slide without looking like a pasted rectangle.
Print. Transparent PNGs work in print design programs. The transparent region becomes the paper colour or whatever backdrop the designer chooses.