Now AE is smart when it comes to crafting with basic machines but if you need to put more than one item into a specific part of the machine (such as inscribers) or you need to use more than one machine to craft an item then a buffer chest is needed.
You can add an interface onto a chest for a craft, if the craft needs 2 machines. From the buffer chest you can use pipes to direct certain materials to certain machines, once crafted the finished product can be sent back to an interface and into the system. This will fulfill the craft and not clog up your stem.
You will need a buffer chest if you are filling ducts with fluids and crafting within an interface.