I'll make an exception for authors who want to tweak or improve their work.
Sometimes directors have to give into the limitations of their medium -- time, money, meddling producers, etc. If they want to revise it later to meet their original vision, more power to them.
Also entirely possible it was a cash grab; Skywalker Ranch needed a new wing or something.
For a positive example, Ridley Scott has a bunch of edits of Blade Runner. The huge improvement over the narration-riddled theatrical release was huge, and the pathological edits around hints towards Decker's replicant-ness are just kind of fun, in that different people are likely to have different views on the subject depending on which cut they watched.
Sometimes directors have to give into the limitations of their medium -- time, money, meddling producers, etc. If they want to revise it later to meet their original vision, more power to them.
Also entirely possible it was a cash grab; Skywalker Ranch needed a new wing or something.