Advertising largely just reminds you a product or service exists. If FedEx never advertised, you may not think much about them, but just having funny ads or having their brand visible might be just enough to get someone to chose their service over another. In some sense, this is good, because it makes these companies compete harder for your business, because one or two bad experiences can do far more damage than 100 funny ads or on-time deliveries.
And there is something to say about FedEx being the first company to come to mind when you brought it up.