Graphics.CopyTexture is one of the few ways that Unity allows you to draw to Cubemaps without the use of a Custom Render Texture.

It also allows direct access to mip levels, allowing world creators to construct Custom Reflection Probe shaders with correct metallic and diffuse mips.

Third, CopyTexture can be placed anywhere in the render chain similar to Blit, ie. after an individual camera renders, vs a Custom Render Textures that only update at the start of the frame.

This also makes handling Texture Arrays and texture cropping far easier.