A placebo doesn't have therapeutic value but it doesn't mean it has no effect at all. And the Meningitis vaccine doesn't have therapeutic value in the contest of Covid-19.

In fact, there are even placebo surgeries. They put you to sleep, cut you open, and stitch you back without actually doing anything inside your body. It is far from nothing, the anesthesia and the incision are real. It is important because you want to compare the effect of the actual operation to everything else that is involved in a surgical procedure.