John Gill Commentary Exodus 30:10

"And Aaron shall make atonement upon the horns of it once in the year; with the blood of the sin-offering of atonement once in the year shall he make atonement for it throughout your generations: it is most holy unto Jehovah." — Exodus 30:10 (ASV)

And Aaron shall make an atonement upon the horns of it once in
a year
On the day of atonement, as the Targum of Jonathan, and so Jarchi and Aben Ezra explain it; and the atonement here referred to seems to be an atonement for the altar itself, see (Leviticus 16:18Leviticus 16:19) and as the altar of burnt offering was first expiated and then used, (Exodus 29:36Exodus 29:37) so it seems the altar of incense had not only an atonement made on it, but for it: and this was done

with the blood of the sin offering of atonement ;
by sprinkling the blood of that offering upon the horns of it, as we learn from the afore mentioned place; and this shows that Christ's mediation and intercession is founded upon the virtue of his blood, and the efficacy of his atoning sacrifice, see (1 John 2:1 1 John 2:2)

once in the year shall he make atonement upon it, throughout your
generations ;
which proves the insufficiency of all legal sacrifices of themselves to take away sin, since every year, as the apostle observes, there was a remembrance of it, (Hebrews 10:3Hebrews 10:4)

it is most holy unto the Lord ;
either the atonement made on the day of atonement, which was a most holy part of service, and pointed at the great atonement made by the most Holy One, the Son of God; or this altar thus expiated, and devoted to sacred use, was reckoned a most sacred one to the Lord, and so was to have nothing offered upon it but what he ordered; with which Jarchi agrees in his note,``the altar is sanctified to these things only, and not to any other service.''