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There is a background prop with a sky/cloud photo behind the airplane. If you look closely, much of the realism comes from the reflections of those clouds on the planes surfaces. If you did the composite of that in post, the plane would look fake.

sometimes you render more parts so that you can then experiment with colors and do some small changes.

if we dont use the ENVsphere we can not have physical correct reflections. what i would do for a render like you posted:
-render the plane plus the ENVsphere. that way i get real reflections
-turn of ENV sphere. make BG white and turn of all lights. render. i have now a matte.

now i can go in photoshop and open this all together. i duplicate the layer and on the second layer i erase with the  ''matte''  the background. now i have two seperate pics. i have now full control. if maybe teh lighting was not good enough i can now do some small changes to really make sure that the render fits with the BG. i can now replace the BG with a similar pic that also has clouds.
sometimes we dont get very big and clean  pics for 360 enviorments. here i could now blur it a little and made some simple DOF.  it all depends.
it is extra work but you have more control for last changes. at the end of the day its not about how fast you can post the render but how good it is.

ILM and Digital Domain are always trying to get almost everything in the render. they dont like to do  touch ups . but they have better tools.