If a player makes the varsity team just because they played the year before, players would not work as hard to keep their spot because if they know they don't have to earn it. I see it as a way to weed out the weaker players and/ or make them work to become better each year.
Not sure if that's true or not. Fair question.
If the player was a starter on varsity the season before, that player might not have as much incentive to work hard.
But if the player did not start, then they have the incentive to become a starter. And the returning starter must produce or she won't be a starter for long.
It could also be argued that a player doesn't feel that seniority is rewarded will not work as hard. Why spend a second year on JV if the coach is not going to give me any credit for that next year when I'm trying to beat out a freshman? Sometimes it is a motivator is a player can see that ''OK, I've got to be on JV two seasons, but if I pay my dues and get better, there is a very good chance I'll be on varsity as a junior and might even start as a senior.''