The Hebrews want to return to the ceremonial law of the Old Testament. But that’s not Christianity; it’s apostasy.
So the writer - Paul, I think - explains how this path would be wrong, disastrous and in chapter 7, biblically illogical. The argument: remember that mysterious fellow named Melchizedek (1)? He was the king and high priest of Salem (2) - perhaps a precursor to Jeru-Salem. Unlike everyone else in Genesis, he has no genealogy, no beginning, no end - he pops in, Abraham pays him a tithe, and then he pops out again. Nothing more is heard of him until Psalm 110, when David speaks of a king and priest to come, after the order of Melchizedek, not the Levites, from Aaron.
This is important, because in God’s covenants, the priesthood goes with the sacraments that administer that covenant - things like the sacrifice of bulls and goats (9:13). So when the priesthood changes, so do those sacrifices. With Jesus came that change of covenant and priesthood: he became the guarantor of a better covenant than the one the Levites administered (22), dying for our sins as High Priest, once for all. And for all time, because he is risen: death cannot stop him from serving as our High Priest. Thus he constantly intercedes in heaven for us, enabling Him to save us to the uttermost (25)! So why go back to what is obsolete?