My understanding of this, and I’m not a developer so that is second-hand, is that there were some significant performance issues with mounted zvols discovered in the early release versions, so that is part of the motivation. The other purpose, which is indicated in the UI help text, is to allow Incus to fully manage the instance volumes, which in turn exposes them for Incus’s native backup/restore and snapshot functionality.
Work is in progress to expose these features in the UI.
Currently it does not, but I believe there is also development work in progress to allow you to select a volume and then make it the root disk or move it to the root disk at least, so an additional volume mount will not be needed.