A common functional ingredient that helps to keep the other ingredients together, stabilizes and thickens the products. It also helps water to mix with oils and dirt - a useful thing for cleaning the skin and hair.
If you are a chemist geek you might want to know that it’s made from the mixture of Cetearyl alcohol (a fatty alcohol - the nice type of alcohols) and ethylene oxide. The numerical value at the end of the ingredient name marks the average number of ethylene oxide molecules added. 20 seems to be a common choice.
It’s normally used at 1-3% concentration.