I've had it invaded and used against me. Even where it wasn't something I felt was inappropriate. Even when it was wildly misconstrued. I've had open book situations both where a partner did look and, in my current relationship, my partner has the ability but doesn't look.
I've snooped and I've had info unintentionally shared before they wished to share it.
What I've found in all these instances is everyone I've known who super closely guarded their privacy, did so because they could not be completely above board. I'm not even going into an outright wish to hide dishonest dealings or a need to protect sensitive client info in what I'm talking about.
It was that they either wanted it for the possibility of doing something they knew others would be hurt by. Like a security blanket hiding a bunch of what if situations that may or may not ever come to pass. Or because when they did share private info, they wanted to be able to color what was going on to show them in the best possible light. I think this is where the control aspect comes in. We like to control the impression we make. The ego desires to present ourselves in the way we wish others to think of us and this isn't always who we really are.
The thing I've found about snooping is it only leads to pain for the snooper. All you find is what you'll find in anyone even yourself. We struggle to show the dark corners of who we are; they are dark because WE don't accept them any more than we know others will but it is an undeniable part of who we are. The power struggle between wanting to know all parts of our partners and letting them have the power to come to it in their own comfort and time.