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
If your surfaces, after knitting, create a closed surface (i.e., the outside of a block), then you can use the create solid function.
However, if your surfaces do NOT create a closed surface, then you can't use "Create Solid". You have to thicken to give the surface some thickness which creates the solid.
The analogy is this:
If you used surfacing to create two halves of a sphere separately, then knitted them together, you could create a solid when you knit them. In essence you would create a solid sphere (like a marble or a baseball).
On the other hand, if you did NOT use the "Create solid" in the knitting command, and instead used the thicken command after knitting, then you would get a hollow sphere with a material thickness of whatever you used in the thicken command (like a tennis ball).