It assures that you have a buffer zone between you and another faction you're attacking. A good technique that can lead to some great multi-army battles is to get a Military Access treaty, then go beat up on the faction on the other side of your ally. When the faction you just started attacking comes after you, retreat into your ally's land and make a stand near one of your ally's armies.

This does two things: first it gets you some great epic battles, and second it assures that your ally will likely remain your ally, because you're suddenly the good guy, helping them protect their land from invasion.