Why is it necessary when performing an experiment that a large quantity of tests and trials are performed?

It necessary when performing an experiment that a large quantity of tests and trials are performed because observations tend to have errors. There are random errors, which are errors in measurement, and there are systematic errors, which are errors in procedure or calibration. Both have to be considered. Performing the experiment more than once allows one to estimate the varience of the results, to reject erroneous results, and to more accurately describe how the results fit the theory.