That by standing on a taxi rank, they are plying for hire in accordance with the terms of their licence. The duty to take a fare (up to the distance limit) if a taxi is plying for hire is part of the deal. It is certainly unusual and possibly unique as a trade, in so much as you (largely speaking) have individual self employed providers, who are required to provide a very regulated and uniform service (in terms of vehicle standards and fares) and only really have a lot of choice about where and at what times they will offer it. In return, the trade has a monopoly on 'plying for hire' in the street, although that monopoly has arguably been less valuable since minicabs based on telephone / two way radios became a thing, and even less so with mobile phone use and app based stuff. Licensing authorities also can impose a maximum number of hackney carriage licences in an area, to try and avoid an area being flooded with taxis (and the trade being unsustainable) - although I don't think London does this, and authorities can be required to prove that there is not 'unmet demand' as a result. Licensing authorities can not apply quantity restrictions to private hire / mini cabs (Mayor Boris Johnson did ask government for powers to do this, but not sure anyone has taken the idea that seriously yet.) In an ideal world, maybe we wouldn't start from where we are now. Hackney carriages (taxis, not to be confused with private hire vehicles or mini cabs which came along later and weren't regulated until a heck of a lot later, and later still in London) have been regulated since the 1830s, and while things like the 'bale of hay' are urban myths, it is all a bit messy. There are occasional efforts to review it all (sometimes suggesting doing away with the divide between hackney / private hire) which usually end up pissing everyone off and getting quietly shelved.