As you correctly point out on a fundamental level unions are just workers of a particular skill organizing to address issues these workers want to see adressed, and by them bekng many they strengthen their bargaining position. Unions can devolve into dysfunctional bureaucratic entities, but so can literally everything else where people come together for a cause — so that is no argument against unions.

Every employee is replaceable, banding together with others that might replace you instead of letting them play you like a fool is a power move.