The logic of carrying a backup gun has become well established. Guns can malfunction or break, so being able to transition quickly to a backup is helpful. Sometimes, a backup can be handed to a trained but unarmed companion.
Hideout or "last-ditch" guns serve a slightly different purpose. Tiny and easily hidden, they enable a person to remain armed without the potential opponent being aware of it. While their accuracy, stopping power and ammo capacity are always minimal, they offer a fighting chance in case authorities or other thugs suddenly manifest murderous intent. That fighting chance is especially valuable for people who can't hope to overpower others by martial art skills or brute physical force.
A hideout gun can be hidden in innocuous locations. A single playing card can cover up some examples.
Kel-tec P32, Ruger LCP and similar 32 and 380 pistols are on the larger side of the hideout pistol spectrum.
Browning Baby and the earlier Colt Vest pocket are all smaller, though heavier than a P32. They compromise on stopping power and controllability even more than the P32, but they fit into smaller spaces. Carried covertly, they can be very handy for people who are initially forced to comply with an order to come unarmed to some government location (though a combination of a metal detector and xray pretty much limits the options to non-magnetic knives and batons) or suddenly discover some other deadly threat at a place where full-sized weapons would be inaccessible.
Hideout weapons, anyone? Fighting with micro-pistols against men with military rifles is truly a last-ditch effort...but it is better than going without a fight. In American reality, getting caught up in a riot or a VT-type mass murder event is more likely...and even a 25ACP would go a long way to discourage the attackers from loitering.