Thus, in some kinds of infectious disease, the body is entirely capable of clearing the infection on its own in most cases as long as the subject isn't suffering some kind of immunodeficiency (e.g. all the strange diseases that HIV patients succumbed to before antiviral therapy was introduced). Studies like this led to the movement to reduce antibiotic treatment for minor infections that are very unlikely to lead to issues like sepsis etc, given the deleterious side effects on the human microbiome and so on. A placebo study might also reveal significant problems with toxicity, such as liver-kidney damage to the subject caused by a problem with the antibiotic under study.
Likewise with vaccines, careful study with placebos could demonstrate that the vaccine under study didn't prevent transmissibility of the infectious virus any better than the placebo did. Notably this was a problem with all the Sars-Cov2 vaccines, none seem to have prevented transmision from an infected person to a vaccinated person. Why wasn't this seen in the clincal trials, one wonders?
As far as depression/mental health studies, placebos are often readily detectable by the patient, which is a big problem in today's research into the 5-HT2a receptor drug research (eg psilocybin, LSD, DMT, mescaline). They've tried to get around this by giving subjects drugs like methyphenidate and methamphetamine as the 'placebo' (because the subject clearly can tell when they've been given a powerful psychoactive substance), but this is not a usual placebo, it's an alternative treatment.