Stuff that is currently in page.ready is whatever should run when the document is ready.
However a long-time known issue is that these are not re-run when an article is rendered after the document-ready event (for example when using ajax/live preview).
As a result collapsible elements, sortable tables and what not don't work when previewing articles.
By having these custom MediaWiki event based, they will run whenever needed, even multiple times during a page's live.
Other events may be the view of a diff. Several gadgets to ajax-patrolling that show many diffs on the page over time. Other gadgets that enhance a diff view (eg. gadgets that annotate the diff itself, or gadgets like Twinkle that add toollinks for the user names on both sides of the diff).