My theory:
I think the devs are using garrisons as an experiment. They want to see 2 things: How can such a system be technically implemented and how do players respond to it and view it. My hunch is that this was supposed to part of Titan (now Overwatch) and Blizz just decided to add something conceptually similar (Garrisons) to an existing IP. If you read this article about Titan, you can see that that game also had a profession system where you'd work in one profession in some home base and then get out into the world to fight (aka PvE/PvP). And thats what garrisons are: your own profession base with a few other perks/mini games.
Garrisons were already in development well before the Overwatch reveal and i guess that if they tried to implement it in the actualy Titan project, it would have taken more development time to add such a system in that new IP.
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Another reason for adding garrisons could possibly be a test to add one or two more professions/systems. gathering proffessions are boring so they have given them to all characters to deemphasize them as 'alt proffessions'. We have lumbering now. Expect to see lumbering added as a profession in the next expansion.