Many Designs of Experiments end in failure because their responses cannot be measured quantitatively.
For example, product inspectors use a qualitative method of determining if a product passes quality assurance or not. This is not efficient in designs of experiments as a pass/fail is not accurate enough.
Replicate to Dampen Uncontrollable Variation: Replicating a given set of conditions many times gives more opportunities for one to precisely estimate responses.
Replicating also gives one the opportunity to detect significant effects such as signals amid the natural process uncontrollable variations, like noise.
For some projects, variations such as noise drown out the signal, so it is useful to find the signal to noise ratio before doing a design of experiment.
