I think that it’s something that snowballs. Someone will start to read another persons emails out of insecurity. But rather than quenching that thirst, it just grows, and so every opportunity they will be checking up.
Amusingly I installed the tracking software on my partners phone, in case she lost it. She constantly leaves her phone on silent, and the tracking software will allow you to login and make the alarm go off … and locate the phone. Because the phone is always on silent, there have been times when she is due home from work, that I’ve tried to call and fine out what time she’s due back, but to no avail. So I logon to the tracking, can see that she’s enroute, and put the kettle on for her return!
She has access to my tablet and phone, and will use those devices. I don’t think she has ever snooped on my mail. Most of it is boring stuff, but some of it is conversations with female friends, including former partners. They’d probably make her jealous, and no doubt lead to questions … but from my perspective, there is no intention to cheat ever. I have a motto: “If you aint happy, then get it sorted before you look for something new”.