How is the UUID for a user used by macOS?
The UUID (aka GeneratedUID, so Google that one instead) is used by Open Directory Services on macOS X. You can see this and more using the macOS app Directory Utility. Read more the book Apple Training Series: Mac OS X Directory Services.

Ownership and Permissions problems would result from a change of UUID. Not just regarding files and folders, but other data such as that in Calendar.

PS: I no longer see the Create New button in High Sierra.