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.
For example, in 2016, 90% of the first $856 of your AIME was added to your PIA, plus 32% of your AIME from $856 to $5,157, plus 15% of your AIME over $5,157. The amounts are added up to come up with your PIA. To see your entire covered earnings history, you can check your annual Social Security Statement.