What is the advantage of running a Ripple masternode? There is one thing that I cannot stop wondering about:
Why would someone run a Ripple masternode?
Before you mark this as a duplicate, I understand how the transaction fees work in Ripple and also the mechanism of distributed consensus.
The point is - for other currency types, the usual rewards for providing services keeping them alive are transaction fees. If the masternodes don't collect any fees, what is the reason to operate one and bear the associated costs?
The same reason someone would run a bitcoin full node. If you want to transact on the network, you either have to run your own node or use someone else's. If you choose to use someone else's, you are relying on them to give you a proper view of the network and run their server reliably. If you run your own, you are independent. You also help improve the stability and reliability of the network for everyone.

Strictly speaking, you could just run a node that's not a validator. But there is almost no additional effort to running a validator -- it's literally just one extra command you enter once. It's nothing like mining.

If nobody is using the XRP Ledger or deriving any value from it, then it makes no difference who validates or, for that matter, whether anyone validates at all. If there are people using the XRP Ledger and deriving value from it, then they have a stake in ensuring that the network continues to operate reliably and that its rules continue to be the rules that make the network useful to them. This acts as incentive to run a validator, considering that there is no additional cost.