When designing an electronic device, you want to make sure that it won’t go in smoke under unexpected circumstances. You never know what may go wrong during the device’s lifetime, primarily if it is powered from mains. The small circuit between your battery and device may protect it from mistakes like reverse power polarity.
His lists start with a simple series diode, which is cheap but may help protect from reverse polarity. Like any other, it has a downside – voltage drop (0.7V) that may be a pain for low voltage devices.
The list continues with more smart solutions like Crowbar diode and fuse, which avoids voltage drop but ends up with fuse blow on reverse voltage.
Using MOSFET is a more thoughtful way of protecting circuits. It gives a smaller voltage drop – mV range with the suitable MOSFET selected. This one is a common way used in many circuits.
And of course, There are always specialized IC’s that provide way better protection. Like LTC4365 protects the circuit from reverse polarity but also from under and overvoltage. So it is your choice how much you are willing to pay attention to this part of the circuit. It is probably a “must” for expensive projects or end products where different things may happen.