Some friends like to brush oil on their barbecue food. After seeing this, some staff at barbecue sites recommend this practice to other friends who come to barbecue; as a result, everyone does it, charring the meat without knowing why they're doing it.
After being grilled over charcoal, meat tends to dry out easily and doesn't look good. Brushing a small amount of oil on the grilled meat can improve its appearance. However, this method has been abused, with people brushing excessive amounts of oil from the start. A group of ten or so people can finish an entire bucket of oil. It seems as if brushing less oil would make the grilled food taste bad. In reality, this practice both pollutes the environment and contaminates the food, resulting in blackened barbecue items.
In fact, the right approach is: only when the grilled meat becomes very dry should an extremely small amount of oil be brushed on it to enhance its appearance. Barbecue food that has been properly handled doesn't require oil brushing.
Most barbecues don't need oil brushing.
If too much oil is brushed on, it drips onto the charcoal and turns into smoke, which contains harmful substances. This both pollutes the environment and harms health.