Why cook with the lid closed?

Recently, I bought my first kettle grill, thinking it was just a normal grill but round in shape. I haven’t done much grilling in my life, but every time I’ve done it (or when family or friends have), it’s been pretty simple: light the charcoal, wait for the grill to get hot, and then start cooking. However, in all the tutorials I watched about using a kettle grill, they put the lid over it as if it were an oven.

Why not just cook with the lid off? I can understand using the lid if the weather is bad, but I can’t see any other reason.