A constitution is not needed by the country, but rather by its population. To understand how and why it is needed, we must harken back to the "olden" Roman times (archaism intentional, purely for stylistic purpose).
Rome, like most ancient civilisations, did not have written laws. Whenever there was any dispute, the Senate, formed by the elderly patricians, was called to action. This was a very conservative system, not only were the Senators elderly and not elected, but they were members of the upper class. Their role was to know, interpret and apply "customs" when the dispute must be settled. This was a tremendous power, that they could use almost at whim to assert the privileges of the upper class when patricians and plebeians (the people) were in dispute.