Either for a monument of the victory; or rather, for sacrifice, as the next words imply. The same was the first altar, though he had occasion to do so oft ere this time. So this is quoted as another evidence of his neglect of God and his worship. It is true, Saul sacrificed before this, as at Gilgal; but that was upon an old altar, erected by others.
Related Commentaries of 1 Samuel 14:35
1 Samuel 14:35
35 And Saul built an altar unto the LORD: the same was the first altar that he built unto the LORD.