Because we live in a world where there are hormone disruptors, it is sometimes necesary to suppplement.....There are many things in our environment that mimick estrogen like pesticides, chemical residues in furniture, air pollution, birth control, tap water, plastics etc.... The end result is too much estrogen and of the wrong kind. We have receptor sites in our bodies where hormones basically latch on...kind of like boats in a marina-they have their own place to park but during the day the boat can move around freely......If too much estrogen takes up the progesterone spots then the progesterone cannot work and the whole cycle is now messed up. When we supplement with the right progesterone(not what is prescribed in a doctors office) we slowly start to take back the receptor sited for the correct hormone.
This is the most simplest explanation. Read the book What your doctor may not tell you about perimenopause. The name encompasses anyone who has ever had issues with their cycle and how hormone disruptors work adn why progesterone is a good thing. Feel free to ask any more questions.