This battle I had prompted this question.
In the above battle, Fire Blast missed. Twice. A simple Flamethrower would have ended the battle much sooner.
+-------+----------+------------------+
| Power | Accuracy | "Average" output |
+--------------+-------+----------+------------------+
| Flamethrower | 90 | 100% | 92.8125 |
+--------------+-------+----------+------------------+
| Fire Blast | 110 | 85% | 96.421875 |
+--------------+-------+----------+------------------+
Factoring in that accuracy, your average output is barely 3.5 Base Power higher than that of Flamethrower, and Flamethrower isn't prone to sudden bursts of luck.
(Personally I use Flame Burst – you lose 10BP for it, but the ability to hit the other opponent for 1/16th of their HP, bypassing everything from Substitute to Protect is pretty sweet. Can't tell you how many times I've splashed out a Pokémon that Protected! Great for breaking Focus Sash too!)
I also calculated Heat Wave, factoring in hitting two targets your average overall Base Power expectancy on that move is 66-ish per target, so 132 overall. Not bad!
So, why do people use Fire Blast and related moves? Is that little bit of extra power really worth it? Overall, how likely is Fire Blast to one-shot something that Flamethrower would two-shot?
Best Answer
Well, this is heavily due to the style of battling. Whether someone likes to play a riskier battle, like I think there's a saying that goes like if 'the more you risk, the more you might gain' (and of course, the more you might lose).
Re the answer above. It depends on whether someone feels the extra power but trade off accuracy is better. Some people feel they have a lot of luck, and rarely see it miss, some people would make the Pokemon hold a Wide Lens in an attempt to restore some of the lost accuracy.
That one might be considered too broad. There are various sets (in terms of stat distribution), various Pokemon (typing, base stats, abilities, natures) and various conditions (weather, items, buffs/debuffs) to take into consideration. And 1HKO are not the only things to consider. Fire Blast could 2HKO a Pokemon where Flamethrower would require 3 hits, or 3 hits for Fire Blast against 4 for Flamethrower.
You might want to read this smogon article where parts are discussing about Fire Blast and Flamethrower. It's based on Gen IV data though, but still something you might find interesting.
For instance, let me quote those lines: