Do ImageOptimize Volume Variant Transforms replace Craft's default Asset Transforms?
No, they do not replace Craft transforms. They are in addition to. And in fact, Image Optimize uses regular old Craft Transforms under the hood.

Image Optimize will do three major things for you:


Every image transform you do, whether it be via Craft image transforms, or the Optimized Images field will be run through a variety of image optimizers that you have installed, including making .webp variants
An Optimized Images field makes it easy to create an image srcset for doing responsive images
You can entirely replace the transform method from your local server to a service like Imgix or Thumbor


These all operate independent of each other, but they can work together as well.