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.
A W-2 transcript includes information from your annual W-2, which your employer filed with the Social Security Administration. After processing your W-2, the SSA sends the information to the IRS. You can get your employment or wage history from the IRS via Form 4506-T. When completing the form, check box 8 to order the W-2 transcript. On line 9, state the year of each transcript you are requesting. Transcripts are available for the current year and the three prior tax years on a single Form 4506-T.