Speaking as someone who has lived with my wife for over 10 years and where we can each access each other’s phones (for reasons of administrative convenience), neither of us have ever “snooped” on each other.
So when I hear of people taking advantage of features to hide chats from their partner it makes me wonder about the psychological health of either the relationship, one, or both of the partners.
There are absolutely psychologically unhealthy controlling partners who “snoop” on their partners unreasonably dictating what is and isn’t allowed. And at the same time there are also unfaithful partners who are having the kind of conversations with other people that they really shouldn’t when they’re in a committed relationship.
Only other reason I can think to hide chats are risqué group chats with friends posting arguably inappropriate content, but again, if your partner is snooping on this and then getting controlling, that’s not really healthy.
Finally, I will admit I sometimes use incognito mode on my web browser at times (but never for conversations), so perhaps I’m a bit of a hypocrite.