Essentially, I want to know if the copywriter can or should help us craft our 'why'.
There's a lot of really good stuff here already, so I'll do my best to be brief:
The most successful brands figure out why they're in business and then they look for an intersection: the sweet spot where what they believe and value overlaps with what their target audience(s) believe and value.
Gary Halbert used a great thought exercise with his marketing students: "What advantages would you want to have to sell more hamburgers than your competitors?" He'd let them throw out answers like "high-quality beef" and "a great location" before he'd interject and say that the only one that he needed was a hungry crowd. In other words, you can totally nail your USP and still go broke trying to sell hamburgers in a vegetarian neighborhood.
So you start with the why (a la Sinek), then move to your audiences (your hungry crowds), and then move to that intersection of beliefs and values.
Then you craft a story and develop high-level brand messaging around that sweet spot.
This process works best as a, well, process, that happens with a cross-functional team at your organization—typically 2 4-hour sessions in-person. But you can also do an abbreviated version and get it 90% right (that is, good enough).
Hope this helps,