Repeatability is key to any scientific discovery. There are a whole lot of steps to most experiments, and a whole lot of places where error can sneak in without being noticed. If you repeat a process three times and get the same result each time, you can be much more confident that the result is real.

And it's not just experimental error you're worried about. The thing you're studying could be influenced by all sorts of things. Repetition also helps you get a feel for how much variation there is in your result, and therefore in your sample.

To ensure that the original results were not a fluke or mistake.