What is the difference between a ready-assembled and flat-packed 19-inch cabinet?

Functionally, very little once both are assembled correctly.

The ready-assembled cabinet arrives built, with the door, side panels, mounting profiles, cable-entry covers, locks and bonding components already fitted. The flat-packed version arrives as the same basic cabinet construction in pieces and needs assembling before use.

One practical advantage of the flat-packed version is access. In pieces, it can pass through openings that a fully assembled cabinet may not. A loft hatch is a good example: the finished cabinet might have no realistic chance of getting through it, while the individual panels can be carried up and the cabinet assembled in position.

You’re choosing between factory assembly and doing the assembly yourself, not between two different grades of cabinet.

The flat-pack version hasn’t sacrificed structural integrity for the privilege of fitting into a flatter cardboard box. Once assembled correctly, it’s intended to perform the same job as the ready-assembled version.