The Type XXIII was designed for coastal operations. That role made it a serious threat to coastal convoys and Allied supply movements across the Channel. In the counterfactual comparison here, it was the more relevant boat for threatening D-Day support than the Type XXI. Type XXIII construction quality was also generally better, although the Type XXI set a low standard for that comparison.
Notable Point · 1:31:15-1:32:06
Why Type XXIII boats threatened Channel supply
The Type XXIII's coastal role made it especially suited to attacking convoys and supply routes across the Channel.