deepfakery
When a model is first started it is a blank slate; it has no idea what the data will be. The pretrained models have already learned a variety of face shapes, alignments, masks, color information, etc. When you use a pretrained model you are essentially shortcutting the process by starting with a template for lack of a better term. Someone else has done part of the work for you. Choose a pretrained model with settings that will work well on your system and use that for the base of your deepfakes.
Another advantage of using a pretrained model is that you can choose a model with very high settings and still get a result in a reasonable amount of time. For instance, if the pretrained model will only run at batch size 4 on your system, normally it would take weeks or even months for you to train the model. However by using the pretrained model you can again shortcut the process, still running at a low batch size, but having the pretraining done by someone with a much more powerful system and higher batch size.