It allowed them to intercept ships to check if they were slave trading. While they were inspecting the ship, the British officers could use the pretext to find other things wrong, ie smuggling, piracy, desertion, ect. Then they could seize your property or arrest you in some cases. Sort of like how American cops can use a busted taillight to pull you over, then arrest you if he find drugs during that.
The navy did also free a significant number of slaves this way too so it wasn’t just an abuse of power, but it also signaled to everyone that Britain was the dominant naval power in Europe now.
I agree, German kids could have resisted, there was the White Rose and shit, and let’s not forget the thousands of leftists thrown into the camps. It’s not like people didn’t know what they were doing was wrong, they just didn’t want to take the risk of opposing the regime like the peers did. And to forgive them, punishes those who did resist. That’s why Nazi collaborators deserve a swift execution along side the true believers.