Most engines have a series of overboard drains usually located around the 6 o'clock position on the engine. These drains allow engine components & gearbox drive pads to drain away minor oil & fuel leaks. Over time these leaks stain the cowl. The thrust reverser does a good job of spreading it everywhere.
Another common issue is when servicing the oil tank from small cans it can be easy to spill some and it pools in the lower cowl area. The thrust reverser does the rest.