If you craft something, and hover to the result, but instead of pressing the left click, you press your hotkey button instead, the resulting item would go into the hotkey you pressed. For example, you craft some planks. You put the log, and you hover your mouse to the resulting planks. If you press 1, while hovering on the planks, the planks will go to the first hotkey slot. However, if there is already an item on the first slot, the planks on the result slot would not go anywhere, and you will need to press the left click instead. On previous versions, the item that is already on the slot will get replaced by the new crafted result, but now, it does not. Same on Bedrock.