Rehearsing answers to common questions is helpful preparation for a job interview, but the experience lacks context - there’s no one to respond to your responses. A mock interviewer can ask you for clarification. They’ll pose new questions based on your answers. They provide a sounding board so that you not only practice your responses but also learn from them.
Even if you practice in a mirror, there’s nuance to your interview performance that you aren’t aware of and can’t adequately critique. Body language, tone, passivity (or aggression), and nervous habits are just a few. You may be giving up too much information or not answering questions thoroughly enough. What’s at issue is that you simply don’t know what you don’t know. A mock interview provides a neutral observer to help you see areas for improvement.
It’s good practice to record your mock interview. Afterward, you can review the highlights. Your interviewer can walk you through what you did well and what was lacking. You’ll have the opportunity to critique yourself as well, an essential step for internalizing critical lessons.