Because other scientists and science in general rely on their collegues' research which in turn allows development of our knowledge on the given subject. The more dont-to-earth reason may be safety. If someone performs an experiment without knowledge of its true results it might result in some danger to the safety of those perforning it without knowledge of all the risks.
Honesty is essential in science because it ensures the integrity of research, allows for the replication and verification of findings, promotes safety and ethical standards, and helps prevent scientific misconduct. When results contradict predictions, honest reporting fosters learning and contributes to scientific knowledge. This practice builds trust within the scientific community and supports future discoveries.
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