There are bike power meters that measure exactly the wattage one can put out. Why did the author bother calculating drag coefficients and such? A 40km time tri

There are bike power meters that measure exactly the wattage one can put out. Why did the author bother calculating drag coefficients and such? A 40km time trial takes about an hour to finish, at about 250W sustained ( 250W/h = 900kJ ), or about 22.5kJ = 5.3 kCal per km.