Coffees prepared with some standard home machines (e.g., non-paper-filtered methods like moka, Turkish, or even some espresso machines) have something on the surface. It looks like oil from the beans.
Filter-based brewing techniques like drip or Aeropress do not result in this oil film on the surface, in my observation. The filter is probably the reason, as it can block the oil and just let water get through.
This oil, if confirmed, is what contains most flavour compounds, so I guess we want to keep it. On the other hand, it is aesthetically nicer to have a clean surface for the coffee drink, which means to act such that this oil is excluded at brewing time.
I am not sure there is a tradeoff here, as "clean" surface coffees are very good (if prepared well), which means the flavour compounds did get down to the drink. So perhaps the surface oil is just that does not participate to the flavour. Any information on this topic?