The Levites were charged not only with this task, but with guarding the tent of meeting. As they had seen when their own brothers/cousins offered unauthorized fire on the altar, approaching the tabernacle without the proper respect for God could result in death. Their job included not only maintaining that right relationship with God for themselves, but ensuring the safety of others by guarding the tent from intruders.
Each clan had an assigned spot to guard, forming that circle around the tent that we drew in our diagram yesterday.
God set apart the Levites as an offering for the firstborn sons of the Israelites, all those who had been born since the Exodus. It was an offering of their life in service, rather than one of giving up their life. Those who were spared during the final plague of the Exodus had been redeemed already through the sacrifice described in Exodus 13.
God made a distinction, as He had in Exodus, saying that "every firstborn that opens the womb belongs to me." In the New Testament, we see that Christ is ultimately the firstborn. He is the firstborn of all creation as well as the firstborn from the dead (Colossians 1).
Just as the Levites protected the people by interceding between them and God, so that the people's sin would not result in their death; so Christ places Himself between us and God to protect us, to be the substitute for us, and to serve as our priest.