Villiers St, the location of the pop-up urinal, runs from the Strand down to the river at Embankment underground station. You can get the train there, or you can walk across a footbridge to Waterloo station, which is the terminus for trains that serve a big section of the suburbs. Beyond the Strand is Soho and the West End, with lots of theaters, restaurants and pubs. People often head home down Villiers St after consuming a happy amount of alcohol, which may give them both the desire to unload some liquid, and a lack of inhibition that allows them to do so in the street. Most of this happens in the evening--during the day, the same street is packed with commuters and tourists, and the sight of men using the urinal wouldn't be welcome. Some might say it's not a fine thing after dark, but it's better than the same action performed without the urinal. It is, shall we say, a pragmatic solution to a problem. Before anyone asks, no, there's nothing there for women. It's blatantly unfair.
Fun fact: Villiers St was constructed as part of a 17th-century housing development on the estate of the Duke Of Buckingham. The duke agreed to sell his land if streets were named after every word in his name and title, "George Villiers, Duke of Buckingham". And so five streets were laid out--George, Villiers, Duke and Buckingham Streets, and one called "Of Alley". Unfortunately it's since been renamed, but not totally forgotten:
https://i.imgur.com/qI8ln5h.jpg
