The idea of a monster that never stops following you (even while you're asleep) feels like a concept straight out of the late 70's and early 80's birth of the Great American Slasher (from Halloween to Nightmare on Elm Street). The Entity is explicitly defined as an allegorical punishment for sexuality, another creature hell-bent on killing kids who transgress social norms by having sex. It Follows joyously participates in this tradition (calling particularly on the aesthetics of John Carpenter), but it uses these genre tropes more for tense atmosphere than gory thrills.
The tension it creates is one of almost existential anxiety. This fear functions as a symbol which can be read in a variety of ways. Like most great horror films, It Follows works best if you interpret The Entity as a metaphor. Explicitly it represents the restlessness of adolescent sexuality: the only way to get rid of it is to have sex with someone else, artificially reproducing sexual desire as an evil demon haunting the subject. But it also implicitly represents fear of mortality: it's not only trying to kill those it haunts, it also has a crucial dimension of inevitability ("The worst thing is that it is certain.").
These slow, creeping camera moves also help establish the presence of The Entity. It (both the camera and the creature) cannot run, and the ominous atmosphere that this methodical motion creates is enhanced by the possibility that we might be unknowingly looking through the eyes of a monster. This is also unfortunately why The Entity always works better off screen than on: once we see it, it becomes mundane. It's just another something that the characters can run away from. The tension is effective because it hides this presence, and The Entity's supernatural force holds us in suspense.
Moral of the story? Don't have sex, because it'll kill you; but if you do have sex, then have sex with everyone because you're going to die anyway and you may as well have a go of it.
Junesploitation: Day 16 – New Horror!
2014: New Releases