Snooping isn’t right but it can be the only way to find out the truth and leave a relationship without any doubts you did the right thing. Say you’re with someone you suspect of cheating online. You don’t have any proof but something just seems off, you’re SO says all the right things but still you suspect something. It eats away your insides and makes you not sleep. You can leave and always have doubts and think you left someone you loved for all the wrong reasons or you can snoop and find their guilt and leave clear of doubts. Liars are often manipulators who can make you doubt your own feelings.
If you don’t find guilt you’ll always be looking for it, so that situation doesn’t work as well as the one above.