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).
Thicken and coverting solid are totally diff.
Thicken is applied on open surfaces while coverting solid in knit surface is on close surfaces which you can later use shell feature to create skin