Because it makes you a more versatile scorer and more unpredictable to contain, especially in 4th quarters. Teams are stopping it when it matters which is why the Pelicans are an 11th seed, 14-18, 4-6 in their last 10, and that's concerning. I want to see Zion be the best he can be, not just a bunch of window dressing on a non-playoff team. SVG does not need to stunt his potential, by not working on expanding his game. We've seen what happens to guys like Giannis, or Westbrook being just good at what they are an not expanding on it, and how it effects thier game when it matters, and I don't want that for Zion, especially when he's shown he actually has a decent jumpshot. No one is saying he needs to take 10+ 3's a game. Just having a face-up mid range jumpshot, and a legit back to the basket hook would do wonders from his game, and turn him into an MVP level top 3 caliber player.
This mindset is why Ben Simmons is the way he is.