Compensatory Picks -- Can someone explain the justification?

[Daseal]

Why are compensatory picks awarded? Is there a formula for deciding who gets the picks and which picks they get? I know the rules read:

Under the rules for compensatory draft selections, a team losing more or better compensatory free agents than it acquires in the previous year is eligible to receive compensatory draft picks.

However, what metrics decide what you're losing versus what you're gaining. At best this system is unreliable.

I guess I feel like the salary cap evens out the playing field. Big money teams have no advantage in free agency over small teams. If you lose a big player due to salary cap restrictions, you shouldn't get an extra pick -- you had the same opportunity as anyone else.

Can someone explain WHY the league has, and needs, compensatory picks. Also, why didn't we get any for losing a pro-bowl LB while gaining some okay starters via FA, but no real studs.

Edit: Link to the article the quote is from.
NFL announces 32 compensatory draft choices to 15 teams | ProFootballTalk