- people put rooms up on Airbnb and are (perhaps understandably) paranoid about things being stolen. It goes too far if there are cameras in bedrooms though.
- people want to monitor their kids
- general monitoring to catch anyone who breaks in while they are out etc.
Have personally never heard of putting cameras inside for catching domestic violence: I suspect that abusers may be 'smart' in the sense that they'll catch on to this pretty quickly and prevent someone from doing it or manipulate the situation.
Abusers are far far more likely to use cameras against their victims than they are to allow a camera that could be used against them. Technology-enabled surveillance is already a major part of a contemporary domestic abuser's toolkit, this is not even theoretical.
Divorce alone barely ever works, it's not as simple as that. I'd rather not stir the hornet's nest any further as the topic itself is far more delicate than that. It's just an example of why an increasingly distrustful populace might opt to secure themselves through home surveillance.
>are secret cameras a legal way to get evidence?
Depends entirely on the context and where you live. There are no universal surveillance laws. There are also social aspects outside the realm of court why one would opt to do so.
NB: I don't agree with the increasing paranoia, it's just devil's advocate.