Well, the obvious reason was to ease her conscience and to justify her own behavior. If Barry had stopped initiating sex with her (let's say he was having affairs, starting back in Indiana), she'd be almost certain he was having an affair. I suspect that Barry was caught having affairs much earlier and made the usual promises not to do so again and they moved to CO in part because of that situation (they both seem to be attached to romances based ultimately in Indiana, IMO).
Maybe they were both having affairs and both of them had incorporated spying on each other.
So it makes sense to me that Barry would choose an isolated place to park Suzanne (especially if he suspected
her of having affairs. Once a marriage starts to truly disintegrate (especially sexually), partners often get suspicious of each other. She might have believed the isolated place would enable her to keep better tabs on him and prevent more affairs. Since Barry is a suspect, we can sleuth him, and I found at least one SM post tagging Barry and mentioning that he had a Tinder account (I can't recall the name he used).
I can't quite wrap my head around how Barry could have altered the digital information embedded in a picture taken on Suzanne's cell phone but...he is looking much smarter to me than I had thought he was. He could have had a keystroke recorder on her laptop (and also on her phone), he could have been tracking the LinkedIn affair in real time, he could have written down extensive notes or printed out conversations for his Big Binder, so he could imitate Suzanne if he needed to.
But surely the FBI can tell if that information has been overwritten or messed with? Maybe not. But changing the GPS location of where the photo is taken is apparently the hardest thing to do (GPS is never turned off completely when the phone is turned on). However, if the photo were taken on an earlier day, that could be manipulated. But surely the FBI can tell?
The last part of the article below describes how someone can hide their metadata (including location), at least from casual viewers of the photo on the internet.
A Picture is Worth a Thousand Words, Including Your Location
From what I'm reading, there are apps to facilitate this. If the metadata settings are altered in advance, then all photos originating after that app is used will appear to be from a different date.
Which would mean for Barry to pull it off, he'd have had to set the phone to the future - knowing exactly when he'd need to send it, so that the photo appears to be taken around the time of its sending. Then obviously he'd have to get rid of that app and change the settings back to normal. If he used an app to alter the metadata after the photo was taken, the original information would still be there, just not visible but in theory, recoverable.
Why would Barry do this? Only an obsessed, crazed madman would do that, IMO.
He could have imitated Suzanne without sending a picture much more easily.