Why SD/100?

[stoxtrader]

Why do we look at standard deviation per 100 hands? IS there a purpose for this number? Why not look per hand? or per 1k hands?

what is the point of the number? is it to predict future variance? Is it to narrow down winrates?

The predictive power can be powerful and have impactful real life applications for people making decisions about devoting time/effort to playing, career choices, etc... sO i want to understand a bit better why /100 is the norm?