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Congress COULD just authorize issuance of more debt as they pass appropriations, but I do think there are a few practical advantages of having them as separate processes.
One big one is that funding is authorized based on certain estimates of the amount of revenues that the Treasury will be taking in throughout the year.
If something happens and tax revenues begin falling short, the debt ceiling provides a warning sign requiring Congress to reconsider their authorizations, to decide whether those programs should really be funded given this unexpected downturn in revenues.
The debt limit acts as a backstop against unexpected economic scenarios that would otherwise leave the US far more in debt than Congress would have wanted.
