A jointer brings adjacent sides to a square corner, while flattening the side on the table. But you can still end up with a piece that is not parallel in width and/or thickness.
A thickness sander doesn't square corners, but rather brings two opposite sides parallel with each other, as it flattens the side against the drum.
I have both and they see about the same amount of use.
I made a bunch of backed bows before I had the thickness sander, but wouldn't give it up now for anything. Back when, I would cut slats out of staves by using the bandsaw and table saw. Before that, I did a few by flattening staves with a Makita power hand planer, then prep the gluing surface with a toothing plane... but it was torturous in comparison.