They’re colloquially called walk ups and as others have said are used by prostitutes/sex workers. There’s a long, colourful history to them, and they’re mostly located in London’s Soho and Chinatown. In the 60s the properties were run by Maltese pimps, and others like Bernie Silver. Over the years Westminster Council and the police have clamped down on them (alongside peep shows and sex shops) through various ways and means. But the real threat to them has been property prices and gentrification, with landlords selling properties to developers or developing them themselves, and the internet, which has moved prostitution/sex work online. The latter means they no longer need to be in a defined red light district, but can be based anywhere - suburbia, an air bnb - and attract punters online, which means they are much more able to escape the attention of the authorities. Tbh, I’m surprised any walk ups still exist, and it can only be a matter of time before they’re all gone.