[refurb]

Because interventions beyond those being tested are closely monitored. Many are excluded, diagnoses need to be confirmed by specific tests, certain treatments may be required to stay in the trial.

Trials usually run for a set period of time. If a patient gets outside treatment half way through, all data beyond that point (including the final analysis) is tainted. So likely the patient would be disqualified from the study and all their data discarded (and they would just show up as "n=1 patient withdrawn from study due to outside treatment for X").

A good example is an eczema trial. Patient gets drug and is monitored for 1 year. If that patient goes to an outside doctor because the rash got worse and gets a steroid cream, and get better, now you don't know - did the experimental drug help? or the steroid? or both? Was the rash actually worse? Or did the patient just think so? The treating doctor never saw the rash prior, so has no baseline. And they likely didn't score the severity of the rash so another data point could be captured.

It's all very standard a part of controlling variables in a trial.