It's the same as why you wouldn't measure your weight in ounces, but neither would you weigh something very light in pounds - you want a convenient number which is easy to interpret. PC is short for parsec and it's a unit of measurement equal to 3,000,000,000,000,000,000 cm!
Take your annual covered earnings (the earnings on which you pay Social Security payroll taxes), multiply it by that year's index factor and you will obtain your indexed wage for each year you have worked. Next, add up all these indexed wages and divide them by 35 to determine your average wage.
Qualifying for Social Security. Workers do not automatically qualify for Social Security retirement benefits. They must work and pay a minimum in Social Security taxes for at least 40 quarters (10 years) during their working lives if born in 1929 or later. These 40 quarters need not be consecutive.