STP
And what happens when you don’t have STP?
Answer: you get a distribution which leads to statistical matters because external changes are never STP in practice.
STP is merely to provide a reference point. Everything in reality fluctuates and when you have to measure or estimate matters under non-controlled lab conditions, you have to use statistics QED.
I can see you have a high school/college level understanding of chemistry. Maybe you’re a HS Chem teacher. Good for you.
But at higher levels of science, statistical methods are crucial. Case in point: the Heisenberg principle.