Why is it better to conduct an experiment more than once?

The first reason to repeat experiments is simply to verify results. Different science disciplines have different criteria for determining what good results are. Biological assays, for example must be done in at least triplicate to generate acceptable data. Science is built on the assumption that published experimental protocols are repeatable.

2) The next reason to repeat experiments is to develop skills necessary to extend established methods and develop new experiments. "Practice make perfect" is true for the concert hall and the chemical laboratory.

3) Refining experimental observations is another reason to repeat. Maybe you did not follow the progress of the reaction like you should have.

4) Another reason to repeat experiments is to study and/or improve them in way. In the synthetic chemistry laboratory, for example, there is always a desire to improve the yield of a synthetic step. Will certain changes in the experimental conditions lead to a better yield? The only way to find out is to try it! The scientific method informs us that it is best to only make one change at a time.

5) The final reason to repeat an extraction, chromatographic or synthetic protocol is to produce more of your target substance. This is sometimes referred to scale-up.