When you write a story and have a character's POV is that because the character's reaction is more important or because that person can see what is happening to the other person better? I am not sure that made any sense at all. If not just ignore it.
You made sense.

I wish I had a good answer for you.

At this point in my development as a writer, the choice of POV tends to be more intuitive than deliberately chosen. The story comes to me, and I go wherever the Youses Muses Gang takes me.
In a few rare cases, I have chosen the POV and "voice" because I was doing something very specific to the reader. My best example of that is Reversal. I was using a third person external viewpoint, and did not write it from anyone's POV because I was holding back the "reveal". It was interesting to write because I had to stay out of both John's and Aeryn's heads or else I would have given away what was going on.
The other place where I had a long decision-making process about POV is the often-promised, not-yet-delivered Measure of Devotion. In that case, I had settled down into John's POV throughout most of the early chapters, and then realized that there are some parts later in the story where I want very much to work from Aeryn's POV for both of the reasons you mentioned: I want to let the reader know what Aeryn is thinking without using dialogue or action, and I need to have Aeryn be my 'camera' while something is going on with John. Since I know I'm going to want to switch to Aeryn's POV later in the story, I've started swapping every couple of chapters early. That way the reader doesn't go for 10 chapters in John's POV, and then suddenly *ZONK* wind up inside Aeryn's head.
Have you written a story with one person's POV and then the more you get into the story you realize it is someone else's POV that needs to be told? Have you or would you change that POV?
<Runs off to look at a list of fics and ponder this question>
I think the answer is not really. I've definitely had some arguments with the Youses Muses Gang about tense and voice. Foot Falls got written and rewritten about four times before I finally settled on present tense/2nd person, but for obvious reasons, it
had to be told from Aeryn's POV. The Changeling is probably the closest I've come because of that shift in the middle from John's to Aeryn's POV, but I didn't start the second half of the story in John's POV and then have to go back and change it. It just shifted on its own, and I had to sit and stare at it for a couple of
weeks before I decided to let it stay that way.
Good question though because until you asked, I didn't realize how many stories I've written where the POV does shift back and forth. I'd kind of forgotten that Whispers, for very valid reasons, shifts between John and Aeryn ... and D'Argo. I leap into D'Argo a few times, and don't give the reader much warning or help with the transition.

In each instance, I do it for one of the reasons you mentioned, which is to provide an external perspective of what John is going through at that particular moment.