By increasing your ISO your flash will have to do less work to provide the same exposure.

So, for example, if you were shooting at ISO 100 and your flash was firing at full power, increasing the ISO to 200 might allow your flash to fire at half power. Thereby reducing the power required to fire the flash, and thus reducing your recycle time on the flash before you can fire again.

That would give you more shots before your batteries go flat.