How much new code do you think you write from scratch versus using something that you (or someone else) has already written? 

Maybe some variation of it existed in the org, or you had it stored away in your toolbox, or you find a code snippet on the web that you can repurpose.

With the intent, of course, that you know what each line is doing.

Maybe this approach can often be a good/better way to evaluate a Developer during an interview, as wouldn’t that be a majority of what’s happening on the job anyway?

Evaluating, troubleshooting, researching, modifying, retesting, etc.

Rather than writing completely from scratch.

Examples:

Here’s some existing code, please discuss how you would modify it to also do this.

Or

Here’s a problem, show me how you would research and what code you might leverage to solve it.

Or 

We want to do the same thing on this object, as we did on that object, except for X and Y, what should we do.

I would think interview evaluation criteria should be as close to what a day in the life would be for the applicant if they were to get hired.