They produce a "softer" light output with longer duration due to the combustion characteristics. As the material ignites, the "fire" starts slowly at first, producing low light output, increasing rapidly to a peak, then, declining as rapidly as it rose, to low and zero light output. The duration of the light produced can be from 4 milliseconds to 2 seconds, depending on the type and design of flash bulb. This can help to produce photos with more definition with the use of smaller grain film with smaller aperture settings.
From the above website, and would be the reason I would use them.