Unless a treatment is compared to something else, it is not possible to know what would happen without it.
Without a comparison, it is difficult to say that a treatment is the reason something happened.
If people get better or worse after a treatment, you are comparing how they were after the treatment to how they were before it. The problem with such “before and after” comparisons is that you don’t know what would have happened without the treatment.
For example, if a group of people with headaches ate chocolate and their headaches got better, it might seem like the chocolate helped. However, their headaches probably would have gotten better without the chocolate. The best way to find out if the chocolate made a difference would be to compare one group of people who ate chocolate to another group that did not – and to make sure that it was a fair comparison.