Sforno on Leviticus 16:30/ספורנו על ויקרא ט״ז:ל׳

1כי ביום הזה יכפר, the reason why the other factors mentioned just now are required in addition to the sacrificial service is that if the entire load of achieving atonement were to be placed on the shoulders of the High Priest this would amount only to “downgrading” of the seriousness of the sins which is what the term of כיפור is all about. The individual must prepare himself mentally for obtaining forgiveness from his Maker by these additional measures.

2לפני ה' תטהרו, absolute forgiveness, rehabilitation, can occur only in the presence of the Lord, which in turn can be achieved only by personal confession of one’s sin and one’s absolute undertaking not to commit such sins again in similar circumstances. The verse is introduced here to remind us that only the Lord Himself is aware of the sincerity of one’s teshuvah, one’s repentance.