I have a Renzetti Vise, and I have used a Danvise (which I believe is the best bang for the bucks with anyone starting to dress hooks).

True Rotary vices, maintain the hook shank on a level, and even plane, as you rotate the hook in the vise.

Thread wraps; the thread (tinsel, wire, or floss) does not twist, as if you were wrapping the hook shank with a non-rotary vise). I find that I have better control with my ribbing (even spacing), and my wraps maintain an layering on the hook shank. Even floss wraps are smoother, when using a rotary on the hook shank. This also works with dubbed threads, and chenille!

Rotary helps when you are adding a collar wrap, again better control and constant tension.

What I like most about the rotary vise, is the ability to turn the hook to a different presentation; being able to attach material to the opposite side of the hook, with the rotary turning the hook, so you can see what you are doing. I also appreciate when I can tilt the hook, so I can see that my wing is exactly centered on the top of the hook shank, and the wing is not slanted off of the hook shank axis. Also great for working with beards/legs on some patterns.

Primping is easier with a rotary, to eliminate any stray or unwanted material, before removing the fly pattern from the vise.

These are just a few of the many ways a rotary vise, makes the task of dressing hooks easier, and allows you better quality control with the thread and materials.

Bottom line, the rotary vise gives you better control over dressing the hook. It can turned the hook, and it also can lock the hook to any presentation that you need.