Why did the angel appear to David at a threshing floor?


Then the angel of the Lord ordered Gad to tell David to go up and build an altar to the Lord on the threshing floor of Araunah the Jebusite. 1 Chronicles 21:18
            Threshing is process of tearing up the wheat stalks and removing the unwanted part (called chaff) to get to the desirable kernels of wheat to make bread.  This all happens on a flat surface wide enough to handle pounds and pounds of stalks.
            The Bible compares the idea of threshing with judgment day.
 “I baptize you with] water for repentance. But after me comes one who is more powerful than I, whose sandals I am not worthy to carry. He will baptize you with the Holy Spirit and fire. His winnowing fork is in his hand, and he will clear his threshing floor, gathering his wheat into the barn and burning up the chaff with unquenchable fire.”  Matthew 3:11-12
            The unwanted parts will be the unbelievers.  The valuable wheat will be the believers.  Once day Jesus will return and separate the two.
            This threshing floor that David purchased in Araunah became the site for Solomon’s temple.
Then Solomon began to build the temple of the Lord in Jerusalem on Mount Moriah, where the Lord had appeared to his father David. It was on the threshing floor of Araunah the Jebusite, the place provided by David.  2 Chronicles 3:1
            What separates the believer and unbeliever is forgiveness of sins which the temple represented.  The threshing floor is a metaphor for the separation process.