Hardeners are used to cure resins. But this is not a simple case of just adding hardener to the epoxy resin as this may take a long time to cure. For the curing process to be quick enough a different hardener must be used, alongside hardeners with certain additives. These hardeners act as catalysts to speed up the curing progress.
The other case to this is when hardeners are simply used to increase the toughness of the mixture once it sets. Hence, a hardener can be used as either a catalyst in a chemical reaction or as a reactant. If used as a catalyst, the hardener is usually known as an accelerator.