It Follows was a haunting film about some kind of entity that stalked people until it caught them and killed them. The entity could take on the form of any person and could only be seen by those it was following at that time.
Because the entity could be passed on to another through sex, the film was seen by some as a parable about the dangers of sex and sexually transmitted infections. By adapting this into a series it would allow more time to explore the story and delve deeper into the mythology.
It could even go on to finally explore what the entity is, whether it is actually real, if it can be stopped in any permanent way, or even where it came from.