Numbers 3:45 Take the Levites instead of all the firstborn among the children of Israel, and the cattle of the Levites instead of their cattle; and the Levites shall be mine: I am the LORD.
God had spared the lives of all the first born Israelites while they were in Egypt. He spared them because the lamb died in their place. Since the lamb had taken their place and they were still alive God said that the first born belonged to Him.
Then He allowed them to keep their firstborn if they would give Him the tribe of Levi and take care of them. God wanted a people that were dedicated full time to His service. He could have called for their firstborn sons since He spared them but instead He offered them a way to keep their first born. They would take care of His men, the Levites.
Today much the same truth and scenario is played out! God has a people. He calls men to be pastors, missionaries, etc. Most of us get to keep our children. They stay in the family business, go off to college, get a career, and stay close to home.
But God does call some to be totally dedicated to His work. It often takes them from their home, their family, etc. Maybe this helps you understand where the teaching of taking care of the pastors and missionaries comes from!