To pick one example - TCP will get you automatic retry of sending bytes but it knows nothing about the semantics of the layers built on top of it. If your HTTP request to a backend service fails with a 502 - do you try again on the same server? different server? fail the whole thing that was depending on that HTTP request succeeding? TCP can't answer that question for you because it doesn't even know if you're doing a GET or a POST, or what a 502 means.