Altar servers act in place of deacons & subdeacons at the TLM–incensing, giving the priest water to wash his hands, holding up the edges of the chasuble at the Consecration & for genuflections, etc. One doesn’t have to be one to enter seminary, though reportedly it can often encourage a boy to consider a priestly vocation, since he works with the priest “behind the scenes” setting up for Mass and gets to see Mass up close, from the sanctuary.