Your employed, so why should you start looking for a job now? If your skills are a good fit for the job you currently have, and if you, your employer, and your supervisor, all see you as a long-term good fit, then you probably don’t need to answer this question. If you do see the need for a change coming, even if it is not for a while, then there are a lot of very good reasons to start the job search now.

The majority of those looking for jobs are passive job seekers and they can take their time to do research and to reach out to their networking contacts. According to LinkedIn a passive candidate is employed, but not currently looking for a new opportunity and this group accounts for 75% of the workforce.

You need as much time as you can get to be selective, finding the best match for your job skills and strengths and learning about the jobs that aren't listed yet. ,

Potential employers, for the same reasons as you, want to find strong candidates whose experience, strengths, and skills match up with the job description and often they take their time passively looking. Good companies know that good matches result in motivated, happy, employees who help build the company so it really pays to take some time with this decision. Use this time to really look at yourself and be honest about your strengths, abilities and what you enjoy and the dividend you will gain is better job satisfaction and perhaps a better fit in the job you find. 

Potential employers often see a candidate that is employed as a stronger potential employee, still valued by his past employer, not someone who was pushed out considered weaker. As unfair as this is, being in a position where you don’t have to take an offer is an advantage.

Employers will bend to reach out for someone with the best job skills, experience, and knowledge so find the best matches and be sure your resume shows skills and strengths that match those asked for on the job listing. 

Unemployed candidates can easily seem too eager just in trying to sell their experience and skills even in some cases when they aren't the best match. Passion and excitement displayed for the new job can be misinterpreted as just needing a job rather than a genuine belief that a real contribution can be made.

Being actively employed puts you in regular contact with your network and information about changes in the marketplace is often more timely and easier to get. 

A poor hiring decision is a costly mistake, for both the employer and employee so both sides will likely take some time to do their research.