Either, first, Out of respect to it, that it might be worshipped together with Dagon. Or rather, secondly, By way of reproach and contempt of it, as a spoil and trophy set there to the honour of Dagon, to whom doubtless they ascribed this victory, as they did a former, Judges 16:23. And though they had some reverence for the ark before, 1 Samuel 4:7, &c.; yet that was certainly much diminished by their success against Israel, notwithstanding the presence and help of the ark.
Related Commentaries of 1 Samuel 5:2
1 Samuel 5:2
2 When the Philistines took the ark of God, they brought it into the house of Dagon, and set it by Dagon.