Many reasons.

1) Hoisting + mutability

  a = 'lol';
  function a() {}
  a() // error

2) one-liners

  const findByName = (list, name) => list.find(item => item.name === name);
  //vs
  function findByName(list, name) {
    return list.find(item => item.name === name);
  }

3) signaling that `this` keyword usage should not happen