What is visibility:hidden and display:none for? why would you want to hide an element?

Answer 1

That used to be done for SEO purposes, to fill a webpage with ranking keywords that only Google, but not users can see. Now I believe its done so you can keep an element that you dont want to have to reinsert/mod later. I did that with a questionaire field that wasn't ready to go live at some point

Answer 2

Sometimes dynamic forms use them.

Example:
Hide the shipping address fields of the form if "shipping address same as billing address" is checked.