This is a top project manager interview question. The answer shows whether you have realistic expectations from the job you are applying for. It helps the interviewer check if your personal goals align with that of the company.
If you are applying for an entry-level position, it might be too ambitious to say ‘I want to be the CEO of this company’. It helps to research on the employee growth in the company and estimate where you could reach with your best performance in the next 5 years. On the other hand, if you are applying for a senior position, you can talk about how you wish to take the company forward and what your contribution towards that would be.