Yin/Yang is a very big concept in Taoism. To be honest, I don't know where or when the labels "Yin" and "Yang" originated. They are not in the Tao Te Ching, nor the Chuang Tzu. I have read that they were first referred to by these names in the I Ching. So maybe that's it.
Anyway, the concept of Yin/Yang can be found throughout the Tao Te Ching. The second verse begins with this:
Under heaven, all can see beauty as beauty only because there is ugliness
All can know good as good only because there is evil
Therefore, having and not having arise together
Difficult and easy compliment each other
Long and short contrast each other
Voice and sound harmonize each other
Front and back follow one another

(Feng/English translation)

The point being, without its opposite, nothing exists. There can be no such thing as a shadow if there is no light.
Did you ever notice how healthy you feel after you are finally fully recovered from an illness? It makes me feel invincible sometimes. If we did not get sick, we would not notice health at all. We wouldn't even have the word "health". And if there was no alternation of health and sickness, if we only had one or the other all our lives, it wouldn't matter which we had. If we were all sick all the time, vomiting and fevers every day, and knew nothing else, we wouldn't think that it was bad. It would just be the normal state. If we didn't know health, we wouldn't bemoan sickness.
In fact, without opposites, we wouldn't think in the terms good and bad. We would assign every thing, every action, every idea, every everything, the same, neutral, status. But, of course, we wouldn't actually think of things as having that neutral status, they would just be.
And if death did not exist, the word "life" would not exist. We would not think of an endless existence as life. "Life" implies that death will (eventually) follow, and a huge amount of our decisions are (or at least should be) based on the fact that we will not be here forever. If we were going to be here forever, we would do things very differently.
In short, without all of the opposites we have, our existence would be an infinitely long, infinitely boring waste. Everything would be the same every moment. Without opposites, no part of life that we consider worthwhile would exist.

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