The point is a coup is opposite to that of Democratically elected process. "Part of their purpose" means the democracy is a farce, and there was never a choice of the people.
When I was younger, I thought of democracy as the end-all and be-all of political philosophies, since that the progressive march toward of the perfection of democracy is what was emphasized in school. As an adult, I've come to believe that a nation's true value is in to the extent it promotes the liberty and welfare of all its inhabitants, including minorities.
By this metric, my country, the United States, often viewed as one of the world's "great democracies", is doing relatively poorly.
It's the standard problem of having a democracy that votes to end democracy. In America, we get Trump, which is debatably similar...
It's easy to want a dictator when you're in the majority, they hold the beliefs you do and you're probably going to not be the groups oppressed (at first). Raw complete democracy doesn't have a good way of dealing with a majority wanting to repress a minority. So founders (at least smart ones) put checks in to balance that, in the US it takes the form of the Supreme Court and in Turkey they have the military as a final ultimate check.
edit: Source for my original post (can't edit now) of the history of Turkish coups: http://www.aljazeera.com/news/europe/2012/04/201244728146879...