Shampoo is quite a recent invention with the first synthetic shampoo, Drene, being introduced in the United States in the 1930s. Shampoo didn't become a household staple until the early 1960s when products like Breck, Prell, and Johnson & Johnson's Baby shampoo became popular. Prior to that, hair was typically washed with bar soap.
Traditional bar soap is made from animal fat and lye. When mixed with hard water, it left a difficult to rinse soap scum on the hair. This resulted in hair that felt rough and dry. It also left the hair unmanageable. And because traditional soap didn't lather well, there were plenty of marketing opportunities to promote new and improved hair cleansers.
Shampoo is a hair care product that is designed to clean, remove sebum, desquamating skin cells, oils, dirt and sweat from the scalp and hair. It is also designed to beautify. It is simple to formulate a shampoo that will removed unwanted oil and dirt from the hair and scalp but to formulate one that also leaves the hair soft, smooth and manageable is much more challenging. Formulating shampoo is a balancing act between removing enough of the unwanted oils and dirt so the hair appears clean while leaving the hair conditioned and aesthetically pleasing to the consumer.
Although there is no known overall health risk to shampooing with synthetic shampoos, there is little science studying the effects of daily exposure to shampoo ingredients. We do know that over washing the hair can damage the hair itself.
So, if you have a healthy scalp, do you really need to wash your hair with shampoo? For some individuals the answer is a resounding NO. Proponents of the 'no-poo' method espouse that there is no medical reason to wash the hair with synthetic shampoos and that doing so is solely determined by cultural norms. Strengthening their argument is the fact that shampooing has only become a daily essential in the past half-century and the fact that there is little, if any, research proving the health benefits of shampooing as part of our daily hygiene.
Modern shampoos have made it easier to get cosmetic outcomes that are pleasing (clean, silky, manageable hair.) It all depends on how much you want to pay for it and how you feel about using synthetic products.
The selection of effective shampoos on skincare aisles that both clean and beautify is plentiful. Which shampoo you prefer and how often you shampoo is still mostly based on personal preference. Regarding the instructions on many shampoo bottles, Lather Rinse Repeat, personally I don't follow them. I just lather and rinse.