It's great to take these professionals out of the normal element and show they are "real people" too. They have little skills and quirks like all of us, but only get attention for their musical ability. I think the Steph Curry segment might show this the most. Not everyone is a great singer, but we all love to sing along and you can really get a good appreciation of these stars with the segments James does. I think most of his other segments seem pretty staged, but these Carpool Karaokes always seem to have a few scenes that were clearly made up on the spot. The last part of the Ed Sheeran clip above proves that point. They planned the maltesers scene, but that ad libbed melody at the end add to the "authenticity" you describe in this post.