The answer for this will vary based on the company that I am interviewing at. But here’s a framework that I use:
1. I love the Product/Service/Company: I will talk about why I love the product/service/company and how I can connect to the mission/vission of the organization.
2. I love the culture: I will talk about the why I am fit culurally. I will try to map my strengths to company’s culture code and talk about why I am great culture fit and why I would love to work for the team. I would also talk about the data science talent and how I have thrived partnering with teams like this in the past.
3. This is a perfect role for me: I will then talk about why my skills x & y map back to the what the role demands and why it’s a great fit for me that leverages my skills and passion.
I try to figure out where I can add value to the company and how I can be an asset and use all of that research to answer this question.

I just wanted to post this for inspiration. It’s retrospective in nature but still you can adapt the framework used:
William Chen on Why do you work at Quora:
I’ve been at Quora for about a month now as a data scientist. I chose my role at Quora over similar roles at other tech companies. Here were some of my thoughts as I made that decision:
Data is a central piece of the product development process at Quora. I experienced it firsthand when I interned here last summer. Quora uses data to inform and drive every product launch and decision, and the data scientists were the official ambassador between the data and the product. I felt like as a data scientist here I would be at the core of the company and people would care about what I produce.
I’m certain my work will have impact. This was already true when I finished my internship the previous summer. My work on predicting new user engagement was used to completely revamp the new user experience – it led to a more thorough, thoughtful new user experience that helped get new users engaged and using Quora. Plus, as a small company, there is so much to do that I know I can focus my efforts towards product changes and decisions that I think will have the highest impact.
I really believe in the mission to share and grow the world’s knowledge. I spent a summer talking with people and doing old-school research to edit my high school’s Wikipedia article to get it to Featured status. I was frustrated at just how much knowledge and information was locked away in people’s heads or in newspaper archives that were difficult to find. Growing up on various internet forums and using the internet for all my research, I believed in the power of the internet to become the dominant source of knowledge and information in the future. Quora helps make that happen I wanted to be a part of that.
Because I enjoyed using the product, I felt excited to analyze this data. Every time I logged on and read my feed, I felt like I was learning new knowledge and gaining new experiences vicariously. I enjoyed the idea of sharing what I know so it’s archived for the entire Internet, and enjoyed how I was able to use Quora as a platform to get my posts out to the right people. I felt like my experience as a user would enhance my experience as an employee and vice versa. The data just felt so personal and relevant and exciting.
I really liked the people. The team here is top-notch and the quality of insight, discussion, and work proved that to me every day during my internship. Everyone felt unified around the central mission “to share and grow the world’s knowledge”, and I felt I could trust everyone who worked with me that we were all united in a common goal. My teammates were amazingly helpful and knowledgeable and I really enjoyed the mentorship and learning I was encountering.
I knew I wanted to be around people I could learn from and grow with, and I think I found the perfect place for that.