Maybe you already had the following idea: why don’t we put the keys to our wallets and the node with a copy of the list all in the same app? That way we could just have as many nodes as users of our money exist. Looks like a win-win scenario, if we want to have as many copies as possible. Not only that, since everyone would have their own copy, they don’t have to trust someone else’s copy altogether. It’s a great idea, and it already exists.
A full Monero wallet is just that. It’s a wallet app with both the wallet keys (to send and receive) and a copy of all transactions. There’s a catch though. It has a copy of all transactions ever made by everyone in our money network. That’s hundreds of gigabytes. Ouch. If that’s the only option we have, our money would only be usable by people with a lot of storage. That’s not very practical. What about older cellphones for example? What about crappy computers in the hands of people that can’t afford newer ones?
The solution is to split that in two: having light-wallets that only have the keys and store the amounts belonging to them, and nodes on a separate device with enough storage to have a full copy of the blockchain.
There’s also another special kind of wallet that we could have, but to keep it simple, we’ll take a look at them afterwards.