#3 It's an extra brand which allows extra time for more superstars
Being on WWE television is what superstars long for, but they need to be ready for it or they'll never make it. Even if they do make it, all careers end up with a lull or two that may mean the star needs to be refreshed and possibly start things over.
All of that means WWE is even smarter for having NXT and not putting it on television.
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If a superstar is a bit green or not quite ready for the big time, they can learn from the coaches and their peers in NXT. If a superstar comes from Japan and has an entirely different style of wrestling, they can learn the WWE way of doing things in NXT. If a superstar's gimmick has grown stale and they need to re-brand and practice their new persona, they can do that at the Performance Center.
NXT also gives guys like Hideo Itami a good place to work out the kinks while also working through a series of unfortunate injuries. Yes, there can still be bad breaks on the main roster and that was proven by Finn Balor having to relinquish the Universal Championship.