Jamieson, Fausset and Brown on Exodus 30:7-8

7-8. Aaron shall burn thereon sweet incense--literally, "incense of spices"--Strong aromatic substances were burnt upon this altar to counteract by their odoriferous fragrance the offensive fumes of the sacrifices; or the incense was employed in an offering of tributary homage which the Orientals used to make as a mark of honor to kings; and as God was Theocratic Ruler of Israel, His palace was not to be wanting in a usage of such significancy. Both these ends were served by this altar--that of fumigating the apartments of the sacred edifice, while the pure lambent flame, according to Oriental notions, was an honorary tribute to the majesty of Israel's King. But there was a far higher meaning in it still; for as the tabernacle was not only a palace for Israel's King, but a place of worship for Israel's God, this altar was immediately connected with a religious purpose. In the style of the sacred writers, incense was a symbol or emblem of prayer (Psa 141:2; Re 5:8; 8:3). From the uniform combination of the two services, it is evident that the incense was an emblem of the prayers of sincere worshippers ascending to heaven in the cloud of perfume; and, accordingly, the priest who officiated at this altar typified the intercessory office of Christ (Lu 1:10; He 7:25).

every morning ... at even--In every period of the national history this daily worship was scrupulously observed.