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.
How the Social Security payroll tax works now. The Social Security payroll tax rate is currently 6.2%, although it's not quite that simple. First of all, not all wages are subject to Social Security tax. As of 2016, only the first $118,500 is used to calculate Social Security tax -- anything above that amount is ignored. In other words, a worker who earns $118,500 per year pays the same Social Security tax as a worker who earns $1,000,000.