What is the use of a hidden border?

Answer 1

border-style: hidden will override a border conflict. border-style: none will be ignored in a border conflict.

For example, say you have two adjacent table cells. One specifies a border, one says none. If you use border collapse to merge the borders, the 'none' style will be ignored and a border appear. If you use 'hidden' on the second cell, no border will appear on that cell, even with border collapse.