Why do employers ask such question?
Just like a traditional hiring process, employers are looking for someone who is willing to stay with the company long term. Since employers are going to invest on time, training, and money on you, it is important for them to get your commitment and to see if you are a good fit for the position.
How will the employer know you are committed?
Employers are sensitive to the way you answer the question. Note that even the employer interviews you through a VOIP (Skype for example), they have a way to tell if you are sincere or not. They can even tell if you have memorized your answer or you are just reading from your notes. So answer sincerely. Integrity is what employers are looking for.
For more good reads on how to answer this question, read from Virtual Vocation
Resource: Virtual Vocation