Jamieson, Fausset and Brown on 2 Samuel 24:21

21. to build an altar unto the Lord, that the plague may be stayed--It is evident that the plague was not stayed till after the altar was built, and the sacrifice offered, so that what is related (2Sa 24:16) was by anticipation. Previous to the offering of this sacrifice, he had seen the destroying angel as well as offered the intercessory prayer (2Sa 24:17). This was a sacrifice of expiation; and the reason why he was allowed to offer it on Mount Moriah was partly in gracious consideration to his fear of repairing to Gibeon (1Ch 21:29, 30), and partly in anticipation of the removal of the tabernacle and the erection of the temple there (2Ch 3:1).