The principle reason is to place young men on the road to the priesthood. A young man that served at the Gregorian Latin Mass from say 6 years of age till he was 18, knew the mass and rubrics and Latin better than 99% of the priests today.
Makes sense. Then as long as boys are given priority at altar server spots over girls then it is ok to have female altar servers (as long as there is a slot open for a girl that is not filled by a boy)?