The Levites from Zadok"s branch of the priestly family, however, would have special privileges since Zadok and his sons had served the Lord faithfully in the past (cf. Ezekiel 40:46; 1 Samuel 2:35; 2 Samuel 8:17; 2 Samuel 15:24-29; 1 Kings 2:26-35; 1 Chronicles 6:7-8). They would be able to approach the Lord Himself and minister to Him by presenting the sacrifices of the people to Him. They had permission to enter the temple proper, to place sacrifices on the "table" (the altar, Ezekiel 40:46, and or the table in the holy place, Ezekiel 41:22), and to fulfill what God commanded concerning His worship. [source][source][source]
"In every age the sovereign and gracious God has a remnant of those who cleave to Him in spite of adverse circumstances and the mounting pressures of the majority to conform. The Zadokite priests kept themselves from the idolatry of the nation, even though the other priests complied to the idolatrous desires of the disobedient people. For this faithfulness the reward from the Lord will be access to His presence, the privilege of ministering in any and all phases of priestly duty." [1][source]
Allen believed God gave this revelation to settle disputes about the staffing of the post-exilic temple. [2][source]