Hiring somebody is a huge and risky investment. Against the risks and the recruitment and ongoing costs - wages, training, expenses and recruitment agency fees - is the aim and the hope that you will be worth the time and the money and be good enough and hang around long enough to be worth the hassle for more than a few weeks.
Sensible investors and sensible employers do their homework - their due diligence. They want a return on their investment long term. An employer would be daft if they didn’t want to know as much as possible about you before offering you a contract.
This is why the question about why you left your last job is a popular one. You shouldn't fear it - you should expect it, prepare for it and welcome it.
Here are some ideas about how to answer "Why did you leave your last job?":
Be open and honest
Whatever the reason you left your previous job, be honest about it. These are the sort of details that potential employers easily find out when checking references. Do not miss your job opportunity because of an unpleasant surprise or a “small lie” or because of an omission. If you left your last job under less than preferable circumstances portray it in a positive light - explaining how you have learnt from it. If you were fired, it is obviously more difficult than if you were made redundant - but preparing for the question can fix this.
Keep it on point
You should be able to easily answer this question in a couple of sentences. You do not need to give every minor detail of the drama of office politics, the gossip and the back-stabbing colleagues. In fact, you should avoid this at all costs. Do not be negative about your employer or former boss either. Dwelling on your previous job rather than focusing on the future can cause you to lose focus when trying to discuss your potential new job in the interview. It will also come across as negative when you want to be positive. Keep your answer brief, easy to understand and well worded - this requires effort and preparation. Don’t mess about preparing for this question.
Keep it positive
No matter what the reason is you left, keep it in a positive light. Don't bog down your interviewer with unnecessary jargon and pretentious language such as not “I didn’t successfully fit in with the diverse corporate culture.” Speak English, don’t flinch, answer openly and clearly. If you are lucky, you left without an issue, or you merely left as you outgrew the place. Either way do not stress about the question - keep it honest, on point and positive.