A circle is one shape among many. It's wrong for some uses, and being honest about that saves time.
Use a rectangle for banner ads, blog headers, hero images, video thumbnails — anything that needs aspect-ratio control. Use a square for Instagram feed posts (1080×1080), product photos, app icons that ship to platforms still using square containers.
Use a rounded rectangle for iOS app icons (Apple draws the rounded mask) and macOS shortcuts. Use the original aspect for OG images, Twitter cards, LinkedIn shared images — round avatars are for profiles only.
If you landed here looking for a non-circle crop, the right tool is crop image (free ratio) or one of the platform-preset crops.