Now that Christ has appeared “once for all”, and has put away sins, there is no further need of repeated sacrifices for sin.  The law is only a shadow (silhouette; outline) of the good things – not the realities – thus is not able to perfect the worshiper, since the guilt of sin remains. The sacrifices are a constant reminder of sins because the blood of bulls and goats cannot remove the penalty of our sin.  “Therefore, when Christ came into the world, he said.… a body you prepared for me..”  “Sacrifice and offering you did not desire…” 
Although the Mosaic Law required the sacrifices, God determined they were insufficient, in that they could not cleanse the sinner  and by submitting to the will of His Father, Christ offered the better sacrifice, setting aside the old order of animal sacrifice – “…….Here I am, I have come to do your will” , by His sacrifice we are sanctified once for all.
The priest’s duties were never finished, he constantly offered the same sacrifices day after day  and since there was no provision made for seats in the Tabernacle, the priest stood to do their tasks. When Christ offered Himself, He sat down at His Father’s right hand indicating that His work of redemption was finished, His sacrifice perfected forever those who are being sanctified and He now awaits His second appearing for His enemies to be made His footstool 
“……by one sacrifice he has made perfect forever those who are being made holy.”  Christ’s sacrifice provided a cleansing of the conscience for those that have believed in Him providing an internal peace with God, we are being made perfect (holy) in this life, and that perfection will be complete when Christ takes us home to be with Himself.
The new covenant which is now written in the heart or mind of the worshiper, is not an outward cleansing, but a cleansing of the conscience, God no more “remembers their sins” (He chooses to forget) since our sins are forgiven forever.
There is now no further need of the Mosaic sacrifices for sins since those sacrifices were only a shadow or picture of Christ’s eternal sacrifice.