God lays out the responsibilities for the priestly class so that those who are not in the priestly class would not feel the need to get involved in things which they do not understand. (Sorry, a whole bunch of double, triple, and quadruple negatives there.) I want to be sure that you understand this was before they entered the promised land and worshiped in the tabernacle which was mobile and when they found themselves wandering in the wilderness. Still, even at that time, God wanted them to worship correctly so he gave them instructions. He didn't leave them to their own devices, He never does by the way.
Aaron and his family were to be set apart in order to perform the duties that were required of them for the Lord. They were so set apart that they were not allowed to be approached by outsiders. I'm not really sure how that worked, just that as vs.7 states: "any outsider who approaches shall be put to death." That sounds serious.
Notice that Aaron and the Levites do not receive any land from the Lord just these responsibilities and the food that comes from the sacrifices. This is unique to all the other tribes who would receive land. I guess that would be enough to guarantee them security for generations to come. This is to serve, according to vs.31 as: "your payment for your service in the tent of meeting." This is a bit of a step away from our Protestant understanding of the Priesthood of all believers. If you want to know more about this concept see here.
"Protestants believe that through Christ they have been given direct access to God, just like a priest; thus the doctrine is called the priesthood of all believers. God is equally accessible to all the faithful, and every Christian has equal potential to minister for God."