Many items can be created manually by hand. Manual crafting is mainly required in the beginning of a game and is somewhat useful to create small amounts of items later on.
Manual crafting has the advantage that all needed ingrediants are created automatically as long as the required resources are in the inventory. So to create an Assembling machine 1 manually it will be queued till the required Iron gear wheels and Electronic circuits are crafted automatically, then the Assembling machine 1 gets crafted.
The disadvantage of manual crafting is that all needed resources need to be in the inventory and it takes a long time to craft lots of items. Therefore it is advised to automate as much as possible by using Assembling machines for example.
Some items can't be manually crafted like the Engine unit.