your'er standing on a dock in some country that uses the metric system and the tank just topped off at 56.78 liters it's nice to look at a chart and see that it's 15 gallons. Or, when the meter says 18.92 liters and and your 5 gallon jug which you previously meticulously marked at 5 gallons is a gallon short, a printed chart is a big help. I always fill up my Jerry jugs first, check the meter, and then fill up my main tank. The guy in Turtle Bay tried to cheat me out of almost 15 gallons once. I refused to pay the kid more then the agreed price. He went back to the dock and his boss came out in his run-about. We discussed it but when he got mad I told him to being his Panga back out and I'd pump the fuel back into his tank. He didn't like it but took the money and left. I don't mind a small amount of "gringo tax" but, come on, 15 gallons on a 70 gallon purchase? Even I can't rationalize that. After you tell them how much fuel you need they come out in a Panga with a big plastic tank and pump it into your boat. You have no way of knowing how much fuel they put in the tank at the pier other then knowing your boats fuel tankage really well. I have a dip stick that's marked at five gallon intervals for my main tank so I figure I can miss by 1.5 gallons at the most. My dip stick and my conversion chart and I'm good to go. Most people don't have a clue and shoot from the hip. That's fine if you don't mind paying an "idiot gringo tax".