Can someone explain the "bring a 6 pack of beer and the mechanic will suck your dick meme"?

I don't understand. If the beer is worth less than the fee, why would the mechanic work for a 6-pack of beer? If you pay the fee, why would you need a 6-pack of beer on top of that to make sure the mechanic does a good job? Further more why would I want a drunk person working on my bike or telling me about bikes?