If L E D lamps are so efficient, why do the recessed spotlamps have a big metal heat sink?
Efficiency of a light bulb is the ratio of heat produced vs light output per watt. Yes, LED's generate heat, more than a little, still they generate more light output per watt consumed and less heat per watt consumed than other devices. Oh, and to answer your question: to keep from overheating that tiny little junction that generates both.
I am definitely not a pro plumber, but I am a pro crastinator