Deleted member 1487
So that the RN isn't building even more ASW ships and isn't concentrating the whole of its considerable force against what would otherwise be the only arrow in Germany's naval quiver - the submarine.
I'm commenting here while several miles from my library, so I don't have references immediately available, but it seems to me that the Germans couldn't decide what kind of war they were fighting at sea - much as was the case with the Battle of Britain. They committed to building a surface fleet with at least some capital ships, and then used that fleet for very little. It always seemed to me that they spent way too much time in harbor.
On the other hand, any scenario that involves Germany concentrating on just subs and doesn't have the Brits responding appropriately insults England's collective intelligence - and they were not stupid. If Germany was just building subs, the RN's job is greatly simplified - they only have to defend against subs. The RN advantage is maintained; the battle line just looks a little different.
The big problem early on when the British were relearning ASW wasn't that they didn't have the ships, it was that they were misused. ASW assets were running around chasing phantoms in HK groups, which left convoys with 1-2 escorts of dubious quality. Here there might be more rejects guarding convoys, though these escorts are of higher quality, but the problems of technology, strategy, and tactics still remain. Convoys weren't really effective until 1941, merchant ships didn't want to remain in them, escorts relied on ASDIC that couldn't detect surface ships, which is exactly how Uboats attacked (at night) when going after convoys. Individual ships or laggers were ruthlessly picked off. There was a reason that the early war period was called the 'happy time'. And Uboat losses were minimal. More escorts might have helped in this period, but it also means more Wolfpacks early on, which overloads the defenders and increases losses. Besides that the extra destroyers or corvettes might end up with HK groups instead of escorts.
Also remember that the British didn't really even react in their building policy until March 1939 in their building program once Germany renounced the naval treaty, so Germany, if they time things right, can delay a British response to achieve maximum production advantage over the British (though being Nazis I doubt planning much figured into things...).
Still 1939 - early 1941 was the most vulnerable period in the war at sea for the British; a bigger German submarine threat can cause far more damage at this period than OTL, which, although not a war winner in itself, can change things for Britain for the worse during the critical early period of the war.