From my understanding these roles have a tie to the systems of government that have emerged from monarchies yet I really see no practical reason for these roles to be separated. What justifications did people give for including an additional cerminomial office serve to a nation if it effectively devoid of power?
You seem to have answered your own question. There's no practical reason, and (almost?) All states with a symbolic head of state are constitutional monarchies.
If you want a more detailed answer, it's because of the move towards democracy, and both the public belief that one's ruler should be elected, and the belief of those with limited power (parliament, for example, limited by the monarch) that they should have more. Having an elected ruler (ie prime minister) solves both of these problems, but since there's no major, sudden changes, the core system doesn't change, so the monarchy stays.