The Kohanim were commanded to bring fire on the Mizbe'ach. For most Korbanos, the Kohen may use a service vessel to apply the blood on the Mizbe'ach. The Torah says that if the entire congregation of Israel errs and a sin results, then an offering has to be brought. The only Name of Hashem that is used throughout the Torah in connection with animal sacrifices is the 4 letter Name - the Name representing His attribute of Mercy.
Offerings have to be brought without any blemishes.
The unblemished state of an offering symbolizes that when a Jew seeks to come closer to Hashem, he should do so with all of his faculties, with nothing omitted. From then on, the avoda is the exclusive duty of the Kohanim. At that stage the bnei yona are too old and the turim are still too young. With his fingernail the Kohen cuts the nape of the neck. From then on, the avoda is the exclusive duty of the Kohanim .