Every feature you build generates data. Every user action, every AI call, every payment, every error — it all needs to go somewhere structured, or it's gone forever.
Bad schema design is invisible at first. Everything works with 10 users. Then at 1,000 users your queries take 30 seconds. At 10,000 users your dashboard crashes. At 100,000 users you're rewriting everything from scratch.
Good schema design costs you an extra 20 minutes up front and saves you months later.