Prime Minister and President? Why have both?

I don't get why there needs to be both? I know that the President is the chief of command but does the Prime Minister do? I don't get it? Example Russia: Yes Medvedev is President and Putin is Prime Minister but it seems like Putin has the more power [i don't know if it's due to him being the former... show more I don't get why there needs to be both? I know that the President is the chief of command but does the Prime Minister do? I don't get it? Example Russia: Yes Medvedev is President and Putin is Prime Minister but it seems like Putin has the more power [i don't know if it's due to him being the former President]. In the case of Great Britain [i know there's no President] but the Prime Minister is the President whereas the Queen is a figure-head and does have some say in matters...Can anyone help the confusion here? thank you!
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