It's necessary due to the vehicle settings that the APIM controls/has access to. If you bought a an APIM from a non-ST fiesta, you don't "technically" need to, but your Hill-Hold assist may not work properly and you will lose "Sport mode", but still be able to turn traction control on/off. Also, programming allows you to add enjoyable functionality like seeing the climate control on the screen as well as changing the APIM startup to a "ST Performance" boot screen instead of the normal Ford one. Plus many other things. Programming manually is not easy, it's best to pay someone for a custom built APIM file and then flash it using Forscan.
If you are happy with the functionality of the APIM as is, you may not need to. YMMV.
Also, as mentioned you will need a blue fakra GPS antenna or you will see errors and not have that functionality. You may also need to change the USB hub or you will see errors.