Why? Becuase that's our system, we inherited from the brits when we reblled. Rather than starting over with a new legal system, we kept the system of common law and precedent from the English legal system and put our own American spin on it.
I would say it's for judicial economy. pmp testThe courts are busy enough as they are. Why retry a case when the rule of law for a set of facts have already been established. Also the principal praxis testof stare decisis dissipates some legal conflicts before they even take form because the parties know what the consequences will be. So I guess you can say stare decisis decreases the amount of conflict in society on some level.