It wasn't until a little before the last century that synthetics (man made) materials were created. In 1910, it was rayon, 1930's nylon, then spandex, polyester and many more. In the 1950's, "Wash and Wear" became very popular and saved many hours of work. Today, synthetics are everywhere and in almost everything.
In the late 1970's, the researchers at Malden Mills, now Polartec, wanted to create a product for outdoor activities that would act like wool but not itch, smell, or shrink when wet. (Although wearers of wool say that that only happens with cheap wool, many disagree. There is one thing that everyone agrees on and that is good quality wool is very expensive). They were successful in creating a product they called Polar Fleece. Fleece has come a long way since then and keeps getting better as researchers keep developing high performance materials.
I love fleece because it is soft, warm, durable, recyclable, wash and dries quickly, light weight, doesn't wrinkle, doesn't itch, blocks out wind, repels water, doesn't stain, and is awesome for people that are allergic to wool but love the great outdoors. It is also a great material for kids because it has a stretch that gives when they move, run, play, or climb. Kids can also get it dirty and it washes up great. A big negative is that it melts at high heat so you cannot dry it in a really hot dryer and have it stay soft.
My shop only uses anti-pill fleece which resists pilling. It is a higher quality fleece so it is usually more expensive. I also love Polartec fleece which is a great fabric but costs significantly more and hope to use it more often for my solid jackets. I know there is a lot of interest in organic cotton fleece and am currently doing research. I hope to have some jackets made out of it in the future.