Vaan goes through the same thing as Ashe - he lost a loved one to the empire and is driven by revenge - but gets over it before she does, therefore helping her get over it. For all we know, without him there, she could've become a cold-blooded dictator and instigate mass genocide on Archadia.

Also, him being the bystander allows us to get introduced to Ivalice and its political tribulations in a very accessible manner