Understanding when the employer wants you to start is essential part of any job interview. There are of course limitations on your part, such as your cancelation period, which may be different in other countries. This interview question can, when used right, be a great way to move the process forward. It can help you get one step closer to getting the job.
Understand Employer's Expectation: The obvious reason for asking this question is to understand when the employer wants you to start.
Benefit from Unemployment: If you are unemployed or for some other reason available right away, then this is a good way to understand if your immediate availability would be of interest to the employer. On the other hand, if it was important then they would most likely have asked you this earlier in the process themselves.
Make the Employer See You in the Role: One trick to good interviewing is to ask questions to the employer that enables them to already see you in the job. This question can for some people do the trick. It can get you talking about what would be if, which at least leads it in this direction. There are however better questions that lead us in this direction.
All Questions to Move the Process Forward during the job interview
All HR & Recruiter Questions to ask at an Interview