From Noah onward, the altar is where God's people offered sacrifices in worship, atonement, and thanksgiving. Building an altar was an act of devotion, marking a place where heaven and earth met.
Every altar and its blood pointed forward to the ultimate sacrifice — Christ, who offered himself once for all, so that we now come to God through his cross rather than an altar of stone.
The Altar in Scripture
And Noah builded an altar unto the LORD; and took of every clean beast, and of every clean fowl, and offered burnt offerings on the altar.