I paint all that might be seen or hope to be seen. However, I spent a great deal of time on the control panel of the P-38. It looked pretty good unil I put the dash on, the dash covered the entire panel. At least I have pictures of it.
I guess many of us do it because it's there. And maybe that added sense of satisfaction and a little "by the way" story to those who are admiring our work.
On my B-17 I put all kinds of little enhancement details inside; actual maps on the navigator's desk, pen and paper for the radioman, a chart on the pilot's clipboard, & many different things. Most of which may never be seen. But it is satisfying to me to look at the snapshots.
I think many us have our own reasons for "Why" we do or do not paint and/or add details to many unseen or barely seen areas.
I do it, because it's there. 