Creation of CFTR Knockout Mice by Injection of KO ES Cells into a Blastocyst

I am not sure if I am understanding this slide correctly, but what I got thus far is that a KO mouse is generated by injecting the blastocyst with the heterozygous ES cells resulting in a chimeric individual which contains both wild type & heterozygous mutant cells. The blastocyst is then selected and injected into a surrogate female, where she creates chimeric offspring. Now I am confused when they mate chimeric mouse with wild type, what is the point of this? Also, what would result if a KO mouse was not incorporated, what would be the likely result and offspring? Thanks again for reading this and your responses.