In the dark ages
When dinosaurs ruled the earth, most companies couldn't afford computers or the tech priests who ran them.
So they went to what (in this part of the world) they called "consultancies" who would run a couple of big iron dinosaurs & a flock of smaller "mini-computers" (A minicomputer fitted into a single rack, disks were a separate rack) you could buy time of the gear to run payroll, or stock control or whatever, the consultancy operation would be a one stop shop for all your computing needs.
Then as the costs came down and tech priests were more abundant, more & more companies started to run their own gear. They especially liked the fact that the consultancies would no longer have them by the short & curly's ... they would be masters of their own destinies :)
The PC revolution (as stated earlier in the thread) then the networked PC fad of the 90's meant the death of the consultancy model, queue a round or two of in-sourcing & outsourcing and now the trend is cloud computing & managed services, which I'm having a hard time differentiating between and the older "consultancy" model (other than the truly VAST scale of these operations)
So the answer to your cyclical change/structural change question is yes, it's both