The choice of experimental design depends on the number and nature of the treatments under study. It also depends on the objects of the experiment. Another consideration for the choice of the design is the question of available resources. Proper assistance, skill and training are required for the use of complex designs, and in their absence such mistakes in the conduct of the experiment may creep in that it may become very difficult or even impossible to analyse the data.
I believe the main reason we use the experimental design is to understand causal relationships. Experiments generally are high in internal validity as we can manipulate the independent variable and control extraneous variables. Hence, it is a well accepted method across disciplines!
Because expect social science other science demands physical evidence rather then only relying on psychology evidence ,therefore I think experiment is designed for such purpose and even to do a comparative study.
Kaleel Rahman provided an excellent answer. I just want to add that experiments are especially powerful in combination with random assignments where the respective units are assigned to groups by chance. This helps to eliminate selection bias and equates the groups on the expected values of all variables, including those that are unknown.
In an experimental design, the researcher can manipulate one variable and control the other variables. Collis and Hussey (2014) explain that an experimental study is pivotal when the investigator is interested in finding a relationship between variables. Those variables might be an independent variable, a dependent variable, a mediating variable, a moderating variable, a control variable or a confounding variable (Saunders et al., 2016). Saunders, et al. (2016) introduce three types of experimental designs such as classical designs, quasi-experimental designs and within-subject designs.
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