Why do scientists often take several measurements of the same object, average them together, and use the average value as the measurement?

To minimize the practical errors and improve the accuracy.

Explanation:

Scientists take several measurements and use the average of all the measured values to minimize the practical errors due to unavoidable circumstances and be close to the true value. True value is the actual value of a measured parameter without any error, it is the value which is proposed in the design of a certain object. Variations may occur in the measurement due to wear and tear, dust and air pressure. It is almost impossible to create ideal surrounding for the measurement procedure.