Almost never. But if you're gonna find an area of use for it, I would say simply to survive nuke attacks to avoid getting headshotted.
Some examples could be Omrafirs strong beam attack or Gaz'haragoths UE of doom.
On higher lvls, knights manapools consist of thousands of manapoints, which is rarily seen as a resource added to the HP pool which it actually could be.
I think it's much more relevant seeing it as a resource now when strong mana potions has been introduced for knights, but it's still really irrelevant.