Recently read a book called "The Digital Negative." In this book the author explains how LR5 was invented and evolved. In the history discussed it seems that LR took some ideas from Photoshop, but also invented certain improvements to make post processing easier, more logical, and user friendly.
A premise was presented that 95% of your post processing could be performed with LR alone and for that 5% of the time that Photoshop is required Bridge was developed as an interface with some additional organizational tools embedded.
It seems that all that is needed for most is really LR, Photoshop offers some advanced tools that are not available in LR; and bridge is the interface that marries the old and the new, meaning LR (new) and Photoshop (old).
It seems that Bridge has great organizational tools to help sort out workflow say if someone is making a book.
Hope this helps.
Cal