Maybe robot A has a chicken and robot B wants it. But robot B has wheat and robot A wants that. If robot A and robot B can trade with one another right then and there, great, but what if robot A doesn't want wheat *right now*? He won't trade. And maybe when he *does* want wheat, robot B won't want a chicken anymore. With money (i.e. bitcoin) that both robots accept for all goods, robot B can trade *money* for the chicken and robot A can *save* it, and when he wants wheat later he knows robot B wants money (because you can buy *anything* with it) so he has confidence he can buy the wheat with it.