Why D-Day used trained Army units as well as Marines

Should amphibious operations be conducted only by dedicated forces, and why did D-Day use Army units as well as Marines?

The assault mixed specialist forces with Army units trained for the operation; the first waves were not ordinary unprepared formations.

D-Day did not simply send ordinary, unprepared Army formations across the beaches instead of using specialists. Commandos, Royal Marines, and comparable American assault troops formed much of the leading or preliminary waves. The Army units following them had spent months training for their assigned amphibious tasks. The assault used a layered force in which dedicated specialists opened the operation and specially prepared general formations supplied the necessary mass behind them.