Really it's about measuring how likely vaccinated participants are to contracting the virus than those that received the placebo.
A real vaccine, with side effects that are similar to what is expected of the covid-19 vaccine. It is a well tested and safe vaccine and it will protect you from meningitis but not from covid-19.
They are doing that exactly for the reason you are mentioning. Of course, participants are aware of the nature of the placebo they might receive.
wtf?! a placebo should by definition have no effect at all
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.