Interviews - what do you need them for?

Why do you interview people? What are the final goals of that? I am talking about factual goals, not those one which are declared in your company's  policies. In practice I see quite often that "official" goals are "hire an engineer which would be highly effective and production in this very role/position". Good, sounds great. In fact, when it comes down to concrete interviewers the goasl turn into "show how cool I am, show that those candidates are nothing and do not hire him.". Cruel but true. So, by the end of the day we still have an open position, out candidate writes tons of posts on social media and the only guy who is happy  - that interviewer. He is a hero - defended the Company! Well, as for me, I need an interview to understand whether or not this particular engineer good and suites the requirements, could I learn something from him, is he a fat-learned and generally - "do I want to work with him in the same team?". Simply, keep your coolness for real business.