When you take a portrait today you are probably using a flash or a small strobe. But what would you do if you took that portrait 150 years ago? You would probably use flash powder.
It's a nice toy for all the fire loving togs out there that are not really concerned about burning their back yard (or eyebrows). If you take away the fact that you can not actually get a nice batch today (well, at least without having the fed go down on you) I am still not sure you'd wanna use that. It is highly flammable, totally uncontrollable and give out a nice puff of smoke. (And way slower than any modern strobe)