The downside? They’re expensive. Like, really expensive, depending on which one you get. Whether you really need one depends on your budget and how you’re going to be using your ball. For example, I use official match balls for practicing freekicks because they fly through the air really nicely. However, I don’t use them for training because if I lose my ball I’ll be set back $100-$300.
On the other hand, replicas (sometimes called training balls or gliders) are designed to be just like the official match balls but are much cheaper. Their panels are often stitched rather than thermally-bonded and are made of a different material. However, they’re not necessarily less durable than official match balls. So, they’re the recommended option for most players. https://www.youtube.com/watch?v=j2Xn84L3Kcs