What is the purpose of using the secondary antibody in the ELISA test?

Answer 1

It uses two antibodies. One antibody is specific to the antigen. The other reacts to antigen-antibody complexes, and is coupled to an enzyme. This second antibody, which accounts for "enzyme-linked" in the test's name, can also cause a chromogenic or fluorogenic substrate to produce a signal.

Answer 2

It's amplifies the signal and is conjugated to your reporter enzyme. That way when you add the substarte you can measure the amout of color change.