Early in the nineteenth century some more enlightened business owners started funding teaching for their workers. They realised that workers were more productive if they understood what they were doing. This was the beginning of things like the Mechanics' Institutes, and the knowledge that men learned there - maths, physics, engineering - was called "useful knowledge". It had to be "useful" of course or there was no reason to pay for it.
Later in the nineteenth century the workers began to demand a different kind of learning. They wanted to know about why it was that businesses worked the way they did. They wanted to know about their place in the world. They wanted to know about their relations to other peoples and about the human condition. They wanted politics, economics, philosophy. This was the birthplace of organisations like the Workers' Educational Association, which still carries on that philosophy today, both nationally and internationally. And ultimately they were ancestors of the Open University and its open admissions policy.
To distinguish this knowledge from the knowledge they learned on their bosses' behalf, they called it "really useful knowledge".