I worked for a "well-known minicomputer manufacturer" in the Eighties.
We sold a particular system with small, medium, and large disks.
For a few thousand pounds there was an upgrade path.
An order was placed. An engineer was sent in. The back was opened and a switch was clicked.
And the dormant part of the disk, which had always been there, could now be accessed by the user.
The defence was that it made the entry level system more "affordable," and inevitably there would always be an appetite for more data as time went on. You started with a cautious 15mb, and then paid out for two extra chunks of 15mb "if you ever got round to needing that much."