To be fair, it's not "obviously" better, but it opens a new point on the tradeoff curve. For a lot of use cases full autoregression is clearly better, and for some others ful diffusion will still be better.

Autoregressivity has high quality outputs but is fairly slow. Diffusion has low quality output but is quite fast.

This allows you to go in the middle, not as high quality as full autoregression and not as fast as full diffusion, but a balance between both.