It's always good to prepare for interviews as if you were studying for an exam. Take the time to read about the company, go through the job description again, and anticipate yourself to common questions they'd probably ask. Anyway, it doesn't matter how much time you spent preparing for a job interview. When the interviewer says, "Tell me about yourself," the breadth of the question will always leave you wondering how far they want you to go. Do you want my work experience? A quick description of my personality? Both of them? My entire life story? Please be more specific!
And the "Why should we hire you?" question… Well, because I applied, you invited me to an interview, and I would really like to pay my bills. That's real, but that's not considered a good professional answer. You have to explain what makes you valuable, sounding confident but not overly confident. You have to find the exact balance so as not to sound arrogant. At the same time, don't hesitate too much because you can seem unprepared. Ahhhh, job interviews are difficult. I get it.
The challenging part is having to turn honest answers into appropriate interview answers. You can't say you left your last job because the environment was terrible. Maybe it really was, but how does the interviewer know the work environment was the problem and not you? They only have your side of the story, and blaming your current employer could make them think you tend to blame other people when something goes wrong. So it's too risky. You end up saying you were "looking for a new challenge," which is also true, but not the first answer that came to your mind. That's why you need to prepare. You have to find answers that are both honest and professional, and if you have those basic questions thought out beforehand, you'll probably do better.