The Spam Eating is a faster alternative to the in-built mouse version that allows you to eat as fast as possible. It is used when you need to stay at full health.
In order to do that, click on the "Food" word, then press Tab to switch the focus to the food button itself.
After that, you can press Enter on your keyboard, it will trigger the same action as if you clicked on the food icon yourself. If you hold Enter, you will start eating as fast as the keyboard allows.
You can increase the rate of eating by additionally spamming the mouse clicks on the same area.
Be aware that the keyboard spam starts to work after a delay, so you need to start holding it before you need to keep yourself at full HP.
There are cases where you can manually eat to win against something that would kill you on AutoEat.
If you go into the Infernal Stronghold shortly after the Volcanic Cave, you need to manually eat to survive the moves after the stun of Malcs.