What can we learn doing an experiment

Experiments most often begin with a theory (hypothesis). They prove that your theory is correct or that it's incorrect. Learning that something doesn't work is often as valuable as learning that it does.

Imagine you have invented a new fabric that you believe would be better in seat belts than the way they are currently made. No responsible automaker would put them in a car without knowing for sure. An experiment using crash dummies would prove it would save lives if they do work better but it would also save lives if it proved they didn't work at all.