The answer to this question of what Juul vaporizers are used for is straight forward and simple.
Juul vaporizers are used to vaporize liquid nicotine known as e-juice. While nicotine vapes, which is what the Juul is, can be used to vaporize e-liquids, the Juul vape can only be used with Juul brand e-liquid. The reason for this being that Juul vapes are designed to work exclusively with Juulpods, which are the cartridges/tanks that Juul Labs makes for them. They come prefilled with vape juice and they're not designed to be refilled so you get what Juul offers and you're happy with it or you choose something else. It's basically that simple.
Liquid Nicotine Vapes
While liquid nicotine vapes come in a lot of different shapes, makes, and styles, they all have this in common: they're designed to vaporize liquid nicotine blends, which makes them electronic cigarettes (e-cigs).
In other words, the Juul is an e-cig even though it doesn't resemble the standard cigarette-shaped e-cigs that many are familiar with.
Juuls are designed for Juuling, which is to say that they're designed for vaping e-juice. That's what Juuling is (using a Juul vaporizer).