I had always understood that Dresden was a retaliation for the German bombing of Coventry, which had no military targets and was fairly far removed from any. (The account I dimly recall didn't indicate whether the German raid on Coventry was an error of some sort or deliberate.)
Fortunately, the citizens of Coventry and Dresden are in recent decades in solidarity because of their mutual suffering, even agreeing not to count bodies to see which city suffered most. . .
A "purpose" I have also seen was to use Dresden to convince the Russians of what the Allies could do. By then, the thought was, the USSR could extend its march to threaten the West. Better to fry Dresden, to put the Ruskies on notice . . . [Personally, I don't buy that logic . . .]
In any event, the fire-bombing of Dresden was tragic beyond belief. Thousands of refugees fleeing the Soviet Army in the German east went to Dresden precisely because it was seen to be a "free" city. It's famed historic architecture and the city's decision not to militarize made it a logical refuge, which the Germans did not think the Allies would "sink" to bombing . . .
Coventry was, and is, full of factories, especially car factories. It holds the same position in England that Detroit holds here. It was a legitimate military target, insofar as any industrial center is.
The mythos that surrounds Coventry is based on two things:
1. Churchill knew Coventry was to be bombed that night, because the cryptologists at Bletchley Park had broken the codes of the German Enigma machines. However, if he had moved a fighter wing there suddenly, or alerted the civil-defense authorities and evacuated the city, the Germans would have known the code was compromised. So he had to stand by in silence as the city was destroyed.
2. Coventry had many beautiful and historic buildings dating back hundreds of years, most of which were destroyed. The most famous was the 14th-century Coventry Cathedral. Its ruins have been preserved as a memorial, next to the new modernist Cathedral, which I find repellent.
-ccm