I think Mourinho wants Rooney for all kinds of reasons.
A) it will shock English press and commentators and deal a major psychological blow to a nearest rival.
B) likewise the positive press it will generate for him and Chelsea will be enormous given how Rooney is viewed by English sports journalists and commentators.
C) Rooney's development stagnated and regressed at Man U since Fergie seemed to stop believing in him as the fulcrum of the team and stopped building his teams around him and started using him as a sub or playing him out on the wing or in midfield. I think Mourinho is thinking if I make Wayne Rooney feel happy and appreciated, get him fitter and get his hunger and aggression back, if I can get the Rooney of 2004-2009 back then I have the best player in the league on my hands rather than just an above average second striker.
D) the old Rooney above fits the Mourinho player profile of a strong, aggressive, bully of a player who also has technique and end product.
E) Mourinho believes in building a spine of a team using native (English in Chelsea's case) players where possible.