[eternityforest]

It tells the laptop how much power is available. And with USB-PD charging it is used for voltage negotiation which removes the risk of destroying a laptop with the wrong voltage charger, while still allowing chargers to be swappable and interchangeable.

I believe they adapt charging speed to available power in some cases. Without the data pin, what if you wanted to make a a car charger, but the cigarette lighter couldn't support enough current for a full power charger? Or what if you wanted an ultra portable charger?

It's a useful feature for a pretty small extra risk.