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 salary history is useful in assessing your level of responsibilities in previous roles. It is also helpful in preparing for negotiations. If you indicate an ending salary of $50,000 in your most recent job, the hiring manager has an idea of what it might take to seal the deal in the new position. The University of California at Berkeley website indicates that you give an employer a negotiation advantage by disclosing your salary history.