When you buy a ally it gives you attack, defense, spy attack, and spy defense bonus. It also gives you a plunder bonus which help you make more money in ebs. Well start off with some cheap ones, then when you get better buy some better ones. By 500k cs you should have at least spent 1 billion dollars on allies. Therefore you get more money in ebs and it pays off. Just keep buying them more expensive as you get better and you will grow faster. But remember always check to see if they active before buying them or they will never grow. So check their clan eb history or if not in clan ask on wall.