Just notational difference other than presence of mutation..
How is it harder to understand 3 + 6 + 9 + ... + 3n means compared to the for loop? Is repeated addition hard to grasp?
No it's not harder to grasp, just less concise. Summation and Product notation exist for the same reason we don't say "a discernible but subtle level of humidity" and just use "moist" instead - it's more convenient. People can be taught to readily understand "moist" or the summation notation. It's much harder to teach people to read the longer notation more quickly.