Why is it necessary to be able to replicate the results of a scientific experiment

Because it looses it's science-street-cred if it can't be replicated.

Essentially.

If you can replicate an experiment you can prove that the results are not just a stroke of luck or happening by chance, but instead that the variables are causing the results and they will always as long as the factors of the experiment remain the same. That is the definition of a scientific experiment.