I have been studying Chapter 8 of _Neron models_ by Bosch et al. The first part deals with the relative Picard functor. A lot of work is done to make it representable. My question would be why this work is done exactly, i.e.:

_Why is it useful for the Picard functor to be representable?_

I do not have a great overview of this subject, hence I cannot find a use myself.