What does the rebuild script do? Is it a one-time thing for installation or when else would you need or want to run it?
Topic on Extension talk:TitleKey/Archive 2
It rebuilds the title index, which you'd want to do if you notice that search suggestions aren't always as expected, due to the capitalization of the first character of the search string. It should be fine to set up on a cron job, or just to rerun manually periodically from CLI.