The banner is not needed for the website to work, otherwise how would the "decline" button work? They can store cookies, otherwise how would they remember your choice? They can track a functional session just fine, full shopping cart and checkout if they want.

What they can't do, not without your opt-in consent, is track the fuck out of you. Non-functional tracking. Analytical tracking. Behavioural tracking. Tying that tracking to an identity. Selling the data about that identity's behaviour to advertisers, to data brokers, to whoever pays.

The banner gets in your face and loudly prefers you press "accept" because if you do -- $$$$ CA-CHING!!! $$$$ -- they now have your opt-in consent to sell visitor data.