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.
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. If social security or Medicare taxes were withheld in error from pay that is not subject to these taxes, contact the employer who withheld the taxes for a refund. If you are unable to get a full refund of the amount from your employer, file a claim for refund with the Internal Revenue Service on Form 843, Claim for Refund and Request for Abatement.