What is TGA?

Targa dates to 1984 and refuses to die, because game engines and 3D packages standardised on it for textures and never had a reason to move. It keeps an alpha channel, optionally compresses runs of identical pixels, and is simple enough that every tool reads it.

What is ICNS?

Apple's icon container, the thing inside every .app bundle that makes it look like something. Like ICO it holds a set of sizes, but it goes considerably larger - Retina displays ask for 1024px icons, which Windows never did.