Because they are emulating the practice of an "Italian tune-up". It dates from the '50s and '60s, and involves driving on an autostrada at full throttle for a period of time, while watching the speed slowly rise. This practice burned off the carbon that built up in the combustion chamber, and the car performed better. This was in the age of multiple carburetors, incomplete combustion, points and condensor ignition systems, and variable fuel quality. The benefits of doing this today are debatable. "Burning out the carbon" makes some sense in concept, but today's engines with sophisticated engine management systems share little with those engines of old.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue