It's the only way to get to the Midland main line (north and south at Cricklewood), and the GC at Neasden, from the GWR and LSWR systems.
Was at one time slated for electrification to get Heathrow Express trains to the City (via St Pancras and Moorgate), but dropped now we are getting Crossrail.
The western bit (Acton Wells-Acton Canal Wharf-WCML Wembley) will be wired as part of the GW scheme, including the link to Acton Yard (GW).
This was documented in the GW franchise ITT.