For Raiblocks a node is essentially the server/client software. Every XRB wallet is running a node and nodes can delegate to other nodes, so every person that trades on an exchange is delegating authority to the exchange's XRB node.
Aa for why you'd want to run one in lieu of fees, this blog does a decent job explaining that ->
https://fynestuff.com/raiblocks-will-succeed-despite-zero-fees/
He does make a nice point wrt businesses could run representative nodes as a form of goodwill in their communities-- assuming Amazon allows any of them to remain running! Small-timers certainly won't want to be nodes tho, for even $3 per month, so there WILL need to be some actual adoption involved. It seems like a couple of major exchanges are now the main competitive nodes to the original 8.
Our main concern is with some of the attacks, but those are true of most cryptos, and Raiblocks is a bit too clever in design to be just thinking about some kind of underhandedness.