Well, you do know that Node-RED has persistent variables now? They will happily store JSON or anything that serialises to JSON.
That is great as long as you don't want loads of data, especially if it is updating rapidly. At which point, you may wish for a db. Also depends on how critical missing a few seconds of data would be on a crash.
You can also serialise to MQTT but again, this may have performance limitations for large, fast moving data.
So in short, depending on your use-cases, you might not even need a db.
On the other hand, if you are already using something that can store JSON and have sufficient device resources along with sufficient management resources, then potentially a db could give you a performance boost.
You could also look at something like CouchDB and its relatives which uses JSON natively.
Another thing to think about is resilience and horizontal scaling. If you need resilience, the tooling behind a dbms may well be very helpful. If you need horizontal scaling (e.g. replication between dbms instances) then again a dbms would be the way to go probably. It can give you enterprise level performance, security and resilience. But at the cost of resource and admin overheads.