At the same time, The Big Bang Theory has a lot of female fans, and bringing in ladies beyond Kaley Cuoco helped to add storylines the ladies watching the sitcom could identify with.
Truly, the men on The Big Bang Theory had some growing up to do when the series started.
They needed to be able to hold and maintain relationships and Mayim Bialik and Melissa Rauch were the catalysts to help Sheldon and Howard grow as people.