In the video to convert hollow object into solid model Knit-Surface and Thicken tools are used. Knit-Surface tool is combining all the surfaces and Thicken tools is converting it into solid.    

My question is combining all the surfaces & converting model into solid entity can be done alone with Knit-Surface tool only, by check making boxes under Knit-Surface tool. Why was it done separately in the video? 

https://www.youtube.com/watch?v=w9MqW3o4ZLE&t=362s

Time 36.46-36.50