Just wonder what could be use to have friend in other alliance. If this friend is placed on same slice then he could just spy on PvP battle chat. Anything else?

its just allowing you to connect when playing different characters. Most people who play mmos have a bunch of characters they can swap between to get some variety to the game. Even when I played characters on a different faction in some past mmos I would still be in an (Alt Room) for voice com. That way if something came up with my main guild we could be called in to help. So basically you were already playing that way with a bunch of other people and might not have realized it.

Only one mmo I ever played used a single character slot. Most let you have like 7 per server or something. ESO not having servers might use a different system or just cap it at like a half a dozen per account or something like that.