Why do we need to forwardRef to the IndeterminateCheckbox?

[c10b10]

1️⃣

The example for Row Selection from the docs implements an IndeterminateCheckbox which makes use of React.forwardRef.

I can't find any explanation in the docs as to why a ref to the underlying checkbox input needs to be passed down from outside the component.

2️⃣

Where can I read about the documentation about defining a custom hook like the one on line 70 of the example?