Earlier quoted context omitted.
This sounds like something Gogolian bureaucrats would say. "The purpose of filling out form A38 is to have form A38 filled out." Modern bureaucracy usually doesn't introduce requirements for the heck of it. Checking citizenship worked fine before the introduction of ESTA so "checking citizenship" was not the reason the system was introduced or is still in place. It does that as well but that is not the point of ESTA.…
The fact that the visa waiver program existed earlier has absolutely no relevance. Was there a way to check requirements prior to ESTA being created? Yes. But now , ESTA is the way requirements are checked for the visa waiver program. That is the purpose for which ESTA exists and for which it was created. That is ESTA's job. That is what ESTA does. ESTA is not some separate data-gathering or visa-issuing system; its…
The Conference agrees on the need for significant
security enhancements to the entire Visa Waiver Program as set
forth in the Senate bill and to the implementation of the
electronic travel authorization system prior to permitting the
Secretary to admit new countries under his new waiver
authority. The Conference mandates that the Secretary develop
such an electronic travel authorization system to collect
biographical and such other information from each prospective
Visa Waiver Program traveler necessary to determine whether the
alien is eligible to travel under the program and whether a law
enforcement or security risk exists in permitting the alien to
travel to the United States. The Conference believes the
Secretary should check the information collected in the
electronic travel authorization system against all appropriate
databases, including lost and stolen passport databases such as
that maintained by Interpol. The Conference believes that
checking travelers from Visa Waiver Program countries against
all appropriate watch lists and databases will greatly enhance
the overall security of the Visa Waiver Program.
https://www.congress.gov/congressional-report/110th-congress...As you see, the main stated purpose is not the check citizenship but to query various databases in advance.
(For completeness I want to add that nowadays "You haven't travelled to some countries" is another requirement to use the VWP. The ESTA application also asks for things visa applications do not, such as the names of your social media accounts.)