When is this useful?

It's not hidden it's still on the page and someone could select it when clicking init.

Why not use a session variable?

Alternatively, you can just add the attribute Visible="false". That has has the added benfit that if would prevent someone from just viewing the page source and discovering the textbox value.

Actually, instead of Visible="False" (which doesn't render the control in the DOM), you could just do style="display:none;" which hides the control on the page, but still renders it on the DOM, and allows you to do anything with it you desire.

It also reduces the amount of code you have to write to do the exact same thing.