After the flood, Hashem made his first covenant with man; that He would never again destroy all of mankind. The sign of this Brit is the קשת, the rainbow, as it says in 9:61 - את קשתי נתתי בענן והיתה לאות ברית ביני ובין הארץ - "My rainbow that I have placed in the clouds it shall be a symbol of the Brit between me and the earth."
Rav Samson Raphael Hirsch says that there is no reason to think that the rainbow was first created now as some want to suggest. According to Rav Hirsch as well as others, the rainbow existed beforehand but only now it took on a new meaning. Rav Hirsch gives two examples to show that such a concept does truly exist. He points to the story of Hashem and Avraham when Hashem shows Avraham the stars in the sky and with the words כה יהיה זרעך changes the meaning and nature of the stars. The second example, a very similar one, is when Hashem shows Moshe the moon and with the words החדש הזה לכם changes the moon's significance as well. Just as both the stars and moon existed before those points in history, the rainbow existed before Hashem showed it to Noach.
Chazal in Pirkei Avot (Mishna 8) say that not only did the rainbow exist before Hashem showed it to Noach, but it was one of the ten items created at twilight before the first Shabbat after creation. Most of the items on this list deal with or are the cause of future miracles. What is so special about the rainbow that it had to be created along with the other items? Furthermore, if the rainbow is a natural event which happens whenever the sun rays flow through the raindrops, why did the rainbow have to be created specially at this point if the sun, clouds, and rain had already been created beforehand giving nature the ability to create a rainbow on its own?
Perhaps, the answer to the first question can be found within the second question. It is true that nature could have and most probably did create a rainbow before this point in history as we have seen above. However, the specific rainbow shown to Noach by Hashem was special. It was not a natural event, it was a rainbow created by God himself earlier in time to be used here. It was created along with the other items because it was special and was not a regular rainbow. A rainbow made by nature would not have sufficed in this case and therefore a unique one was used. It was created along with the other nine items because of its uniqueness.