Why specialist minelaying submarines remained useful

Were purpose-built minelaying submarines a worthwhile investment before the Second World War?

Yes. They could place dangerous concealed minefields in constrained waters, even though wartime demands sometimes diverted them to other work.

First World War minelaying submarines provide an important precedent, and later boats had similarly strong potential. British minelaying submarines were sometimes diverted to supply missions, which concealed what their intended role could accomplish. In East Asian waters, covertly mining channels and straits could endanger Japanese traffic without revealing that the submarine had ever been there. Mines also avoid some of the accuracy limits of aerial minelaying.