I founded the alliance we are in and I still don't know yet. I am told that it is meant to be an extension of a clan since the member cap for a clan is only 75 but that could always change so I don't think that is the main reason. Every week I try to submit the question "How will alliances work in-game?" to the mail sack but my question has not been picked yet, so no official answer. I am currently trying to get additional clans to join us in our We Are Guardian alliance. Feel free to bump my post. You can find a link on my profile page in the activity side-bar:)