I work for a major US airline. We have over 5,000 pilots. A seniority list is the only way to manage a list of this magnitude. It also ensures that when your number comes up for command you're damn well ready. Seniority list dictates choice of bases, vacations, equipment. The downside to a seniority list is that in a period of stagnation there are no command upgrades.
Of course opportunities exist for those with management aspirations to become training captains and eventually managers. A major benefit to a seniority list, in my view, is that you don't have to kiss ass to get ahead. And, as the old saying goes, those who can fly, fly! Those who can't - manage!