The reason for building them in such massive numbers was that the war effort in the East required engines for the military supply trains. Most of the engines built were built specifically to pull standard loads (1200t or 1650t cargo trains) at specific speeds while not exceeding other parameters.
What one has to keep in mind in this regard is that by 1938, 95% of the rolling stock was built before WW1, and consisted of dozens to hundreds of small series previously owned by the train operating agencies of the German states. There was an effort to renew these with unitary models in the 1920s and 30s, but these only produced some 1500 engines over a space of 15 years due to a general lack of money. Even more so, at the end of the 30s, they found that these were built for the 20t axle load limits of German cargo routes, while most railway lines east of Germany only permitted 18t axle load limits.
Hence why procurement during the war was threefold:
- there was on one side an effort to continue to renew the ages-old rolling stock and replace some war losses for at-home usage - that amounted to maybe 1500-2000 out of the 14000 total engines built during the war, including 400 electric locomotives (E44/E94) of which some were used in West and East Germany until around 1990.
- on the other hand there was the above military supply situation, for which about 7,000 BR52 were built.
- and the third side was the mere fact that any locomotive built during the war was a "Kriegslokomotive" by law. All those (often narrow gauge) engines used in mines, in factories, in ports and the like, in an industry vastly expanding during the war? Those were the remaining 5,000 "Kriegslokomotiven". 1500 alone of these for example were the Köf II small shunting diesel locomotives used extensively up until the 1990s, and which still remain in private hands for shunting operations e.g. at factories.
About 80% of the BR52 locomotives built survived the war btw, most ending up in the Soviet Union, Poland, East and West Germany. In most countries they were retired after 1960 at the latest, in some they then became part of specific wartime reserve stocks (e.g. BR52 that were not regauged and still in 1435mm became a strategic reserve for Soviet Forces in Kaliningrad up to the 90s).