These can be benign, like re-creating a classic Star Trek episode.

They can also be used to assassinate someone’s character and reputation. They make it incredibly simple to create pornographic images and video about someone, based only on photos of their face.

Here is a story about to how to detect them in real life:

https://www.theguardian.com/technology/2020/jan/13/what-are-deepfakes-and-how-can-you-spot-them