Psalm 133:1-2 NKJV
Behold, how good and how pleasant it is For brethren to dwell together in unity! It is like the precious oil upon the head, Running down on the beard, The beard of Aaron, Running down on the edge of his garments.
The psalmist David compared brethren dwelling in unity to the anointing oil on the high priest. Why? True unity based on godly purposes creates an atmosphere for the glory, the anointing and the presence of the Holy Spirit. When God's people gather on one accord, with one mind and purpose, the Holy Spirit will show up.