i think they wish to use the tailed beast to instigate war.also with the power of the bijuu on their side its more of an incentive for them to be hired. moreover, once powerful nations lose their bijuu it they will be more open to attack... thus increasing the likelyhood of war, which increase the akatsukis revenue. once theyve crushed all ot the militaries and monopolize on warfare then they become the most power organization in the world.