These border checks are waived, not abolished. In times of crisis, they can (and were, during the Pandemic for example) reinstated. Countries can impose movement restriction for internal regions as well, but these carry a much higher legal overhead.

Also, Schengen borders are transparent only for full EU residents. If your visa and right of abode is restricted to a particular country, the borders are still very relevant.

Even for EU residents, the borders are still relevant as they demarcate places with quite different civil and penal law codes.