It is needed for basic operation, because unlike with a A-to-B cable, power bricks plugged into a USB-C cable have no way of knowing whether the other side is also a power supply that will short out if you try to feed it power instead of drawing power from it. A couple resistors in the sink is probably the cheapest possible way of signaling this information to the supply. This isn't PD, it's just a basic requirement to allow having the same connector on both ends of the cable.
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I know. You need the detection for the fancy stuff like PD, not for basic operation.
So why does it matter that a resistor is missing on the sink side? It's stupid.
It is needed for basic operation, because unlike with a A-to-B cable, power bricks plugged into a USB-C cable have no way of knowing whether the other side is also a power supply that will short out if you try to feed it power instead of drawing power from it. A couple resistors in the sink is probably the cheapest possible way of signaling this information to the supply. This isn't PD, it's just a basic requirement to allow having the same connector on both ends of the cable.
You already got the answer spelled out several times here in this thread.