> What kind of data lacks any relations whatsoever?

You're right: all data has some kind of relation - when we say "non-relational data" we almost always really mean "loosely related." But some data is much more strongly related than others.

Think of a traditional RDBMS application such as an ordering system where there are strong relational constraints that need to be enforced before committing your data: ie, your order items had damn well better correlate with an order, which damn well better correlate with a user, or else nothing makes sense -- you had better raise an exception and should definitely not store those things in your database if the relational integrity is not there.

Now constrast that with other types of data collection where relational integrity is much more relaxed.

Generally, these examples would be (near) real time data collection - times when you're collecting data with a "collect first, analyze/correlate later" mentality. Specific examples would be a service that aggregates logs from a server farm in real time, or things like tracking user behavior statistics on the web. Or even an onboard computer that collects data from a car's engine - you want to read that data every 50ms no matter what, and maaaaaybe you will correlate it with events (accidents? combustion issues? whatever) later on oooooorrrr maybe not.

There are times where you really don't need an RDBMS, yes. Hard real-time embedded systems are their own bag, I certainly wouldn't want to tell the people who design the systems inside a car how to do their job. As for server logs... they're their own bag, too, although if you decide to roll your own setup there without fully investigating syslog and journald or whatever else, I think you're probably making a mistake here.

But if you're running a website and you have fewer than 100 servers and you think you need to start rethinking the entire relational data model, you may very well be getting ahead of yourself.