It covers the grill itself, serving as both a way to keep the internal temp where you want it and to protect the inner parts from the elements like the retractable roof on a convertible.
If you left the lid off the grill and other internal parts would would get rusty a lot faster. A BBQ lid does both, although a real cover is better as it also protects the lid.
Just like many people keep their cars parked outside, but the outside is protected better if it is usually stored under cover. The trick is to get a cover that doesn’t trap moisture inside.
OP calling the BBQ lid a cover is like calling the roof of a car a cover.
It covers the grill itself, serving as both a way to keep the internal temp where you want it and to protect the inner parts from the elements like the retractable roof on a convertible.
If you left the lid off the grill and other internal parts would would get rusty a lot faster. A BBQ lid does both, although a real cover is better as it also protects the lid.
Just like many people keep their cars parked outside, but the outside is protected better if it is usually stored under cover. The trick is to get a cover that doesn’t trap moisture inside.