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On December 21, 2000 the Legal Immigration and Family Equity Act (LIFE Act) was signed into law. One of the provisions this legislation is the creation of K3 and K4 non-immigrant visas for spouses of US Citizens who are outside the US, and the children of those foreign spouses. These visas were created to allow reunification of families of US Citizens, by allowing the spouse and children to enter the United States as non-immigrants, and filing for Adjustment of Status inside the United States, rather than waiting for Consular immigrant visa processing.
http://www.visajourney.com/faq/k3k4visa-outline.htmlAt the time the CR-1 visa was a much longer process than it seems to be today, so the K-3 was developed to reduce the time that a married couple would be apart while waiting for the visa.
With the CR-1 immigration visas processing nearly as fast, I would think it would be easier to just do the CR-1/IR-1 visa, so that AOS would not be needed.