If you take a typography class, they will drill it into you that unjustified is the norm (and you will spend some time learning how to make pleasant rags), and that you need a VERY GOOD reason to make text justified.
How much of it is convention vs based in measurable outcomes is up for debate (maybe), but at least that’s where most every formally trained designer/visual artist in the west comes from.
How much of it is convention vs based in measurable outcomes is up for debate (maybe), but at least that’s where most every formally trained designer/visual artist in the west comes from.