This question is a critical indicator as to whether you're interested in this company and role in particular, or whether you're simply applying to any role in the field. For instance, it's not a good sign if you say, "Well, I'm interested in social media, so this role seemed like a good fit." Instead, you want to mention specific qualifications of the role itself, and demonstrate how those qualifications align with your personal strengths. Additionally, it's important you mention how you can help drive results for the team.
For instance, you might say, "Through my last role I've learned I'm passionate about creating content for social media. I've managed to grow our Instagram audience by 7%, and with my team I created a successful Facebook campaign that cultivated an increase in sales by 12%. I've followed your business on Instagram and Facebook for a few years and appreciate your brand -- more importantly, I see this role as one in which I'd truly be challenged and able to use my strengths. In particular, I believe I'd excel in the client-facing aspect of the role. Meeting with clients to collaborate on social media marketing objectives and goals is something I believe I'd find exciting and purposeful."