Identifier is just "a piece of common token system can use to operate on same entity.

You need it. Because it's maybe one lone unchangeable thing. Taking person for example: * date of birth can be changed, if there was error and correction in documents * any and near all of existing physical characteristics can change over time, either due to brain things (deciding to change gender), aging, or accidents (fingerprints no longer apply if you burnt your skin enough) * DNA might be good enough, but that's one fucking long identifier to share and one hard to validate in field.

So an unique ID attached to few other parts to identify current iteration of individual is the best we have, and the best we will get.