Hi Status, I couldn't make to TG1 and do not know Tom's motives but I can see purpose of his actions.
Of course I can't, and don't want to, speak for Tom.
First reason is to create independent source of info (in this case ATR number) where you know exactly how it is calculated thus is safe from any misbehavior of third party calculations and open to any modifications you wont to introduce - now or later on. You can use such number to plug in to other formulas that guide your trading with higher level of confidence and protects you from any unexpected changes done by third party.
Secundo, it is a coding exercise. It may be a step toward creating something much bigger to aid your trading. This is small step but moves you forward.
Third - completing task like this brings you sort of satisfaction. A - that you can do it yourself, B - that you became bit more independent.
That said, I don't think it is necessary step for every trader to take.
You can be successful without any spreadsheet at al and, in some cases, it's better to spend time on smth else.
It all depends
