It keeps you from answering the wrong question. Even if the main objective seems obvious, there could be an additional, underlying objective. The interviewer wants you to think out loud, but think before you speak. If you make a statement that is way off-base in an interview, the recruiter will wonder if he can trust you in front of a client. To turn the question into a conversation.
Nothing turns off an interviewer quicker than a five-minute monologue.
Don't get bogged down in the details. Verbally outlining your response is the hardest part of a case-and the most crucial. Your answer should be as linear as possible. Don't get bogged down in the details. Answer from a macro-level and move the answer forward.