Why transferred Axis submarines brought Japan limited value

How much use did the IJN obtain from Italian and German submarines transferred late in the war?

Only limited use, because Japan was already struggling to fuel and operate its own submarine force where it was needed.

The answer shifts attention from possession to employability. By the time the transferred Italian and German boats were available, the IJN faced fuel shortages and difficulty moving fuel to the bases that needed it. Extra hulls therefore offered only limited value inside a failing logistical system.