How so, and how ia that useful. In general nodes dont provide benefit to the network are are just wasted resources.
It would be more valuable to spend resources on buying coin or developing apps etc
A node will verify blocks and transactions. They will see if the transaction and the block fits the protocol rule. If they don't they will reject the transaction. Lets say we don't have nodes and only miners. The few big miners out there can enforce their own rules.
as long as they are not fake node, for sybil attack they does help otherwise not they are dangerous, you can always check carefully the node that tell the true but it's still possible to perfom it
A sybil attack is hard to prevent. Its hard to find out if the node is sending the accurate information. The more nodes, the harder it will be to connect to the rogue nodes.