There are two wide angles of this question:
Why are you interested in this company?
How you describe your interest in the job?
When an interviewer asks you- “Why you want to work with us?” that means he wants to collect information as:
• Get some details about your goals related to career and what you think about current job profile being offered to you.
• Ensure that you are highly interested in this particular job and will show dedication in your work if they hire you.
• Your research work about the company, position or industry will reflect here.
• They need to understand your viewpoints about preferences and priorities as it will have direct impact on your performance.
Note that, every part of this question must be answered with different approach even for Part time jobs.