Q: Does Pausing or Unpausing items cost credits?
A: Pausing or Unpausing items (the action of turning pause on or turning it off) does not use credits. Repricing/Updating items uses credits as well as checking of items.
Note: As far as why the items could be paused, there are some reasons. If you put items on reserve, set your store on vacation (thereby causing the items to be inaccessible), delete/end the items, etc... then when sku grid goes to revise the item and it can't for those reasons, it will let Sku Grid know and then sku grid pauses the item(s) in the grid to prevent sku grid from trying to make the same changes over and over on items that it cannot change (to prevent you from wasting repricing/updating credits).