Di-ammonium phosphate releases nitrogen in a mash for yeast growth. Many mashes are low on nitrogen and a major cause for stuck fermentations. Adding DAP to mashes allows vigorous yeast activity and faster fermentations.
For example: Juniper berries are very low on nitrogen and definately requires DAP to be added as does pure sugar washes which contains no nitrogen. DAP can be used in conjunction with yeast nutrients for even better results.