the internship - Why did they need to know how to code?

Before I start, I'll disclose that I haven't seen the movie, nor do I work for Google.

Having the characters code may just be something that the writers included for the movie but it would make sense: the best sales people are going to be those that really understand how the product works.

Also, internships are designed to be wide-ranging, to introduce you to many aspects of the company. While you're there, the company is looking to see where you fit-in the best, in case they end up hiring you. Just because you think you're meant for a job in sales doesn't mean that you'll actually end up there. The company you work for may decide that you are a better coder than sales person. If nothing else, showing proficiency in more than one area makes you more appealing to a company.