It's to stiffen the bodyshell when the tailgate is closed, by including the tailgate as part of the structure. It's not just a feature of Subaru.
If you ever park a Subaru Outback on very uneven ground and open the tailgate - you'll realise why when it won't close!
Older estates used to be much worse, often flexing so much you could hear the pins rattling in the aperture as you drove along.
Edited by RT on 19/07/2017 at 21:14
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