But why do we need special Ripple servers at all? Couldn't the system have been designed with a larger collection of nodes in mind and simply done without XRP?
Ripple is a distributed database of bid/ask offers. A distributed system needs to be byzantine fault tolerant (BFT).
Bitcoin achieves BFT by using its native currency (bitcoins) in a proof-of-work scheme. Basically BFT means a malicious node can't do anything which will break the blockchain, because the only way any node can make changes to the blockchain is if it has solved a block (has the private keys to next block of bitcoins). Nodes with private keys to previous bitcoins have to get changes relayed through the node which solves the next block.
Likewise, Ripple achieves BFT by using its native currency (XRP) in a consensus scheme. The consensus scheme relies on Unique Node Lists (UNL), instead of proof-of-work. So nodes can only propose changes to the ripple ledger if they are on the UNL of some other node. And changes will only be accepted if they are signed by the private keys of the native currency (XRP). Nodes which aren't on any UNLs have to broadcast their changes to nodes which are on a UNL, and of course, must still have the private keys to make those changes.
There are probably other ways to provide BFT, but using a native crypto-currency is one of them.
Comments
But why do we need special Ripple servers at all? Couldn't the system have been designed with a larger collection of nodes in mind and simply done without XRP?
Ripple is a distributed database of bid/ask offers. A distributed system needs to be byzantine fault tolerant (BFT).
Bitcoin achieves BFT by using its native currency (bitcoins) in a proof-of-work scheme. Basically BFT means a malicious node can't do anything which will break the blockchain, because the only way any node can make changes to the blockchain is if it has solved a block (has the private keys to next block of bitcoins). Nodes with private keys to previous bitcoins have to get changes relayed through the node which solves the next block.
Likewise, Ripple achieves BFT by using its native currency (XRP) in a consensus scheme. The consensus scheme relies on Unique Node Lists (UNL), instead of proof-of-work. So nodes can only propose changes to the ripple ledger if they are on the UNL of some other node. And changes will only be accepted if they are signed by the private keys of the native currency (XRP). Nodes which aren't on any UNLs have to broadcast their changes to nodes which are on a UNL, and of course, must still have the private keys to make those changes.
There are probably other ways to provide BFT, but using a native crypto-currency is one of them.