Why counter increases and fn is called? How it works?

I solved kata on codewars. Acctually I did this by accident and I don’t understand why this code works. May you explain it?

Kata:

Write a function, persistence, that takes in a positive parameter num and returns its multiplicative persistence, which is the number of times you must multiply the digits in num until you reach a single digit.

39 –> 3 (because 39 = 27, 27 = 14, 1*4 = 4 and 4 has only one digit)

My solution:

function persistence(num) {
  let count = 0;
  const arr = [...num.toString()];
  const sumArr = arr.reduce((res, val) => (res *= val));

  if (arr.length > 1) {
    **// Why? How this line works?
    // Why it doesn't crashes?
    // Why it returns correct counter value and calls the function?**
    count += 1 + persistence(sumArr)
  }
  return count;
}

persistence(39); //3

Why if I do like this, the counter do not save the result:

  if (arr.length > 1) {
    count += 1
    persistence(sumArr)   }