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.
In general, U.S. social security and Medicare taxes apply to payments of wages for services performed as an employee in the United States, regardless of the citizenship or residence of either the employee or the employer. In limited situations, these taxes apply to wages for services performed outside the United States. Your employer should be able to tell you if social security and Medicare taxes apply to your wages. You cannot make voluntary social security payments if no taxes are due. Refund of Taxes Withheld in Error