The double blind study is more important in a clinical trial because it prevents the researcher from influencing the outcome of the experiment.
Double-blind studies improve on experiments that compare the response of people taking a drug (or other treatment) to those who do not. Even though, neither the investigator nor the ones taking the experiment drug know if it is a harmless pill. In this case, the danger of investigator nonverbally communicating their expectation that the drug in clinical trial will work or not is overcome.