For hardware, there is a difference between knowing the schematics and actually editing the schematics. You can have all the schematics you want, but when you try to modify it to suit your needs you need to either remake the schematic or if the original file is shared, edit that instead. As I said, this is my opinion and the ease of modification is generally also part of open source. For a simple part, yes it is possible to remake it. But a complex assembly requires significant effort. Say a roller needs to be spaced a certain amount. You may want to tweak those distances. Before you manufacture it, you check what parts need to be changed to accommodate for your modification too.