I myself can't think of any strategic reasons beyond competition with Google.
It isn't well known but all of these databases of aerial imagery, maps, street directions, speed limits, addresses are all sometimes licensable by third parties, sometimes not, and sometimes at extortionate rates.
How much of your mobile experience hinges on your use of location data? From turn by turn directions, general maps, businesses nearby, mass transit schedules, to friend locations and check-ins. Bottom line, Geographic information systems (GIS) data is a must-have feature for a smartphone.
Google has started raising the price on access to their data, this started a while ago and its been in fits and starts, but Apple can't just 'dump' all map info. Microsoft, same deal $$ for map datasets. So Apple is in the position of its two largest competitors having a way to suck arbitrary amounts of money out of them because they are suppliers of the 'must' have data on a smartphone.
New independent companies that can create GIS datasets can't compete with 'free' from Google and Microsoft for users, and even if they succeeded in creating a decent database and Apple used it, they would be subject to being purchased by Google or Microsoft. Finally there are patents (of course) around the use of map data and constructing maps etc from imagery, which a small company could not defend against but Apple can.
Therefore Apple has to own a GIS database that is at least as good as Google's version so that it can continue to provide a critical feature on their phone cost effectively.