#2 To introduce WWE fans to stars they may not know before hitting the main roster
Not everyone that watches WWE's product is a huge wrestling fan and a lot of them may have never seen anything outside of Vince McMahon's kingdom. Without watching ROH or NJPW or seeing an independent show, they likely won't know too many performers from outside the WWE umbrella.
Do you have any idea who El Generico is? Would you know who was wrestling if Prince Devitt took on KENTA in a match on Raw?
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Fans of wrestling from around the world could easily tell you that Sami Zayn used to wrestle under a mask. They could probably also advise you that a Finn Balor vs. Hideo Itami match would be amazing to have for free on a Monday night.
WWE can use NXT as an introductory ground for new signees after they're given a new name that is owned by Vince McMahon and company. After people are acquainted with them on NXT, they'll be familiar enough with them to pop for the superstar when they hit the main roster.