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. 

 I'm pretty sure that robots and real AI will not give a fuck about money and will just function anf interact differently than any human can imagine. But yes, if robots are just program to copy human behavior and never evolve from it, it might end up like this.
Not sure which option is the most scary one.