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.
What are Social Security Wages? Employers must withhold both Social Security and Medicare taxes (called FICA taxes) from all wages paid to both hourly and salaried employees. But some types of payments are exempt from FICA tax. Thi article discusses what wages are included in the calculation of Social Security wages and which wages are not included. What Wages are Subject to Withholding?