Interview is not really a very accurate means of finding suitable candidates, but when we have no better ways, it's still a very common procedure in a recruitment process.
When you are going to have interviews, that means you are about to give up your current position, or at least you have some motivations to change. In that case, you might be easily in a position that, you think your current position is not very good and promising. However, in this kind of situation, how can you find something good out of your existing position?
Interview, by itself, could be a good way. Why is that?
Interviewers will ask many questions to understand your ability, your attitude, your experience and so on. Through these questions, you might re-discover what you have learned in your current or past positions. That's a good side-effect of an interview. You will easily think all the skills/ability/experience that you have is so common, but that most probably apply in your current company as well. A common skill in a certain place could be entirely valuable in another place.
Even negative experience could be great for the potential new employers, since you have learned from the expensive mistakes from your past/current employers, and these expensive experience will certainly bring benefits to your potential employers. In this sense, even negative or failures could be great.
So all these are the power of interviews.