The point is you defend them every time they get attacked if they are your def ally. You aid them every spy op they attempt if you have an intel alliance. If you want to know what the point of them is, you need look no further than the help button in the game:
http://www.earthempires.com/express/help/in/allies

I know you say you already know, so this part of the post wasn't really necessary, just sayin', it begins and ends there, because teamwork is for the team servers. If I'm playing solo and people start having their allies come at me for taking a retal or attacking them too many times, isn't that unfair to me? Do I need to bring some friends in too? It sounds like we're talking about those other servers now.

One extra note: It's NOT good for you, in basically any case, to get involved in your ally's fight. If you're trying to play for your own individual country's purposes, fighting some guy that isn't fighting you works against that purpose. So you are truly playing a team game by interfering in your ally's battle, and these are solo servers.