In canon, it was described as an implementation of “variable warp geometry”.

In a documentary with writers who worked with Roddenberry they said it was his design choice for all ships. Apparently warp nacelles are supposed to have 40% empty space between them for the warp field to generate. In Voyager they rotated up to exit the plane of the ship.

Basically that. It made for good TV.

Now I'm wondering if Discovery needed to spin or if that was more of the same.