The US Air Force Historical Division wrote a report in response to the international concern about the bombing that remained classified until December 1978. It said that there were 110 factories and 50,000 workers in the city supporting the German war effort at the time of the raid. According to the report, there were aircraft components factories; a poison gas factory (Chemische Fabrik Goye and Company); an anti-aircraft and field gun factory (Lehman); an optical goods factory (Zeiss Ikon AG); and factories producing electrical and X-ray apparatus (Koch & Sterzel [de] AG); gears and differentials (Saxoniswerke); and electric gauges (Gebrüder Bassler). It also said there were barracks, hutted camps, and a munitions storage depot.
The USAF report also states that two of Dresden's traffic routes were of military importance: north-south from Germany to Czechoslovakia, and east–west along the central European uplands. The city was at the junction of the Berlin-Prague-Vienna railway line, as well as the Munich-Breslau, and Hamburg-Leipzig lines.Colonel Harold E. Cook, a US POW held in the Friedrichstadt marshaling yard the night before the attacks, later said that "I saw with my own eyes that Dresden was an armed camp: thousands of German troops, tanks and artillery and miles of freight cars loaded with supplies supporting and transporting German logistics towards the east to meet the Russians".
If you want to learn more you can read the official reasoning why Dresden was bombed in a pdf
here.
We did the same thing to another city, Pforzheim, later that month but you never hear about it. We firebombed the shit out of Hamburg in 43 as well.