You are creating a Custom Data Identifier and wonder why you need to add a Data Normalizer.
The Normalizer ensures that pattern characters are part of the same character type and do not contain any unexpected redundant characters therefore the Normalizer mitigates improper characters before the validation is performed which reduces the likelihood of errors.
The Data Identifier Normalizer kicks in after pattern matching and before the Validator is applied. If the Data Identifier does not have a Validator, the matched content will not be normalized.
When you create a custom data identifier, you must select a normalizer to reconcile the data detected by the pattern with the format expected by the validators.Please consult your Admin guide for more details.