You learn the same amount in normals as you would in ranked. Normals still have a hidden elo which match you up with appropriately skilled people. In addition, you can practice ranked style play in a safe environment with draft pick, or you can practice overall ability with blind picks.
Blind pick will force you to play certain matchups you might deliberately avoid during a draft, which helps you learn champion counters and favorable matchups. Even with mirrors, you can still learn from the other person. If they're outplaying you with your own champion, you have something to learn from them.
And you can take this information and use it for effective draft picking. A trial and error approach to learning champion matchups lets you learn why something is or is not favorable, which can let you reason out other matchups that are favorable or not based on the experience of x against y.
And it's a safe environment to test. Not everybody likes to play FoTM, so normals are a good way to find somebody who isn't FoTM, but you feel comfortable playing on.
You can do all this on ranked, and then ruin your first 100 matches of faster elo movement, meaning even if you do learn from it in bronze, it will take you longer to get out of it, and it will take you longer to move up after it. There's no point in crippling yourself, especially nearing the end of the season, where your last season placement effects your next season placement, when you can learn the exact same stuff in normals.