Placebo trials are important because they create a data point which can be used as a comparison point for the results of the other administered trials. A placebo trial can be compared to experimental set-ups as the control experiment. This ensures that there is a trial that is expected not to change and from there, the different factor that are being changed can be observed and has a reference to quantify the changes.
In terms of ethics, I think that there is nothing unethical with including a placebo-controlled trial in testing. I believe this since participants are only needed to report the effects that they are experiencing, there is nothing harmful coming out of the trial.