Concat Arrays with For Loop

After experimenting with the reduce method to flatten an array, I tried using a similar approach with a for-loop. Can anyone explain why the for-loop doesn't flatten the array using concat? P.S. I know that I can use a nested for-loop with Array.isArray to flatten as well. Just expected concat to work in a for-loop after seeing how reduce flattened the array.

var arrays = [
  [1, 2, 3],
  [4, 5],
  [6]
];

console.log(arrays.reduce(function(arr, elem) {
  return arr.concat(elem);
}, []));

function flatten(arr) {
  var flat = [];
  for (var i = 0; i < arr.length; i++) {
    flat.concat(arr[i]);
  }
  return flat;
}
console.log(flatten(arrays));