In message , Obadiah Jones
writes

Or perhaps I should say 'what was the motivation for building it'?

The principal motivation was commercial competition. The South Eastern
Railway was initially happy with its terminus at London Bridge and its
later extension into the West End at Charing Cross. But in 1860, before
the extension was opened, its great rival (the London Chatham and Dover)
gained powers to build right into the heart of the city with a station
at Ludgate Hill. The South Eastern's answer, primarily in order not to
lose custom, was to build its own city terminus at Cannon Street.

--
Paul Terry