Why does a scientist make repeated observation?

Repeated observations allow scientists to determine the range of variability of a phenomenon. For example, if you take the temperature today where you live, you could make a statement like: "The temperature in Mexico City is 30 degrees Celcius". If you took temperature measurements every day for a year, you could say "The average annual temperature in Mexico City is 20 degrees celcius", etc. The larger number of observations, the more of the variability is captured, and the better representation of the phenomena. Repeated observations can also act as a confirmation or infirmation of a hypothesis.

To put it in Layman's terms, they repeat it to make sure their results are reliable and accurate.