If you want more than 5cm it is a good idea to do it. Even if you don't it is a good idea. It is soft tissue that heals completely quickly and makes the process easier by making you less tight. Paley has mentioned that after healing it is like nothing ever happened to the band. Most surgeons do this prophylactically because it's that safe and beneficial. It is nothing close to an Achilles lengthening which is downright much much more brutal and has devasting lingering effects. This will greatly improve your chances of getting your desired goal.
It is not required, but most people get it because the upside is very good. Even gymnasts who are ridiculously flexible eventually get it half way into lengthening because that's how tight the IT band gets. Better to do it early in the same operation as original surgery than later eh? It's not unecessary, if it was then why would every good surgeon do it? That's ridiculous. You really trust the word of Betz over Paley, Rozbruch, Mahboubian, Parihar, Giotikas, Donghoon Lee?