What is the use of Normalized Email & UserName in .NET core IdentityUser Model?

By my understanding, both fields are there for performance reasons. It's sort of explained in the following thread Normalization on UserName and Email causes slow performance and are used to validate the case insensitive uniqueness of the UserName and Email fields. They are persisted in the database in order be able to create index on them, thus making the lookups by the normalized user name and email sargable.

There is no other reason or usage of these fields.

Because in String.Compare("a", "A"), "a" is greater than "A" based on the binary values. The above compare statment translates to String.Compare(01100001, 01000001). As you can see one value is greater than the other.

Normalization brings the property to UpperCase (I believe) where SQL can index the UserName binary values sequentially without any surprise out of sequence lower case letters and their respective binary values.

This improves performance when searching or querying the database.