Context: Jesus’ once-for-all sacrifice.
Compare with Jewish sacrifice on Yom Kippur (day of atonement); see Leviticus 16
High priest would wash himself in water, put on priestly clothes, enter the Holy of Holies and sprinkle the blood of a bull and a goat on the mercy seat.
BUT (Hebrews 10:4) it is impossible for the blood of bulls and goats to take away sins.
The day of atonement points to the necessity of Jesus’ sacrifice.
Because of this sacrifice, some say we don’t need Christian community; the author of Hebrews has this kind of argument in mind.