Consider the following number in your code:
const oneHundredThousand = 100000;
Large numbers have always been difficult to mentally parse for engineers, so using underscores to make code more readable is a total win:
const oneMillion = 1_000_000; // 1000000
const decimals = 1_000_00.01_02_03 // 100000.010203
The only real rule with numeric separators is that the number cannot start or end with an underscore.