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

I only ask because London Bridge is a mere ten minute stroll from
Cannon Street. Presumably Cannon Street trains have always passed
through London Bridge (apart from those heading towards Charing
Cross). So why go to the expense of building viaducts from the Borough
Market junction, bridging the river, and building a terminus station
at Cannon Street when it's virtually within spitting distance of a
much more significant station at London Bridge? Was it perhaps
intended to extend the line further north at some point?