Magnesium ratchets offer similar strength and durability to aluminum but at a lighter weight, which is why you'll find them on premium bindings. They deliver a really smooth, solid feel when you're cranking down your straps, and they hold up well over time without adding unnecessary weight to your setup.
That said, the weight savings over aluminum is modest—think grams, not ounces. For most riders, aluminum ratchets are perfectly smooth and durable, and you won't notice a performance difference on the hill. Magnesium becomes more appealing if you're building a lightweight splitboard setup or just want top-tier hardware and don't mind paying for it.
Bottom line: magnesium is a nice premium feature, but don't feel like you're missing out if your bindings have aluminum ratchets instead. Focus on overall binding flex and fit first.