js objects and if statements. why this code works without an else and using an else or else if statement doesn't?

I am a very beginner JS "developer" (student) and I have run into a question I haven't been able to solve: why does the value of the repeated letters in my 'helperHash' increase when I am not using an else statement and why this same value DOESN't increase if I use an else statement? My code runs as expected but I am having problems understanding the logic behind this issue...

The code is supposed to return an array with the letters that have 1 or more repetitions in the given str.

function nonUniqueLetters(str){
  var strToChars = str.split('');
  var finalArr = [];
  var helperHash = {};
  for (let i = 0; i < strToChars.length; i += 1){
    let char = str[i];
    if (!helperHash[char]){
      helperHash[char] = 0;
    }
    helperHash[char] += 1;  //HERE! why doesn't this work if inside an else?
  }
  for (var key in helperHash){
    if (helperHash[key] > 1){
      finalArr.push(key);
    }
  }
  return finalArr;
}