Motivation for leaving a current employer can tell recruiters a lot about a person, what their motivations are in life/ in their career and how they react to difficulties in their working life, among other things. Candidates should consider their answer carefully, and have something prepared. This is not an invitation for you to spend the next half an hour of the interview complaining about your current job- this will likely put the recruiter off and can make you come across as unprofessional. Also, if you seem desperate to leave your current job it makes the recruiters question your interest in working for them- do you really want to work here or do you just want to get away from your current job?  

If there are issues at work and that is why you are leaving, you could turn this around as a positive for the company you are interviewing for- "I have heard such great things about _______ and I would love to work for such a supportive company. I want to work in a company that appreciates its staff and supports and encourages them to develop themselves, I have heard that this is the way that you treat your staff and so believe that this is a good culture fit for me." If possible, avoid talking negatively about your current job, if pressed for details as to why you are leaving, be concise and try not to make any personal accusations- you never know who knows who.