What is the logic behind Turkey’s S-400 deal with Russia?

Semih Idiz, Hürriyet Daily News (Turkey)
Turkey, like any other country, has the right to consider its own security needs first. It is clear that it also has a need for air defense systems, otherwise it would not have called on NATO to provide these in the two Gulf Wars and after the Syrian civil war broke out. The question here, however, is a simple one. Is Turkey’s S-400 deal with Russia a political or a military deal? If it is a military deal, its merits have yet to be explained.