The Admin Panel -> Maintenance -> Regenerate page allows you to get some cached database values updated. Regenerating counters allows you to clear up problems that may be caused by directly inserting data into the database, or which in some cases may even result from normal usage. When categories are moved or you change the template code for displaying subcategories, you may need to regenerate categories.

Regenerating categories is the most common process you may need to go through. When you have many levels of categories and you change a category name, it may require regenerating categories for all of the subcategories of this category to properly update their 'parentnames' field to display the changed name in the navigation. If you're using mod_rewrite with category names, this becomes essential to keep the paths up to date -- without the regeneration, you would run into dead links in trying to navigate back up from below the category.

When you move a subcategory to a new parent, the totals will be off until you regenerate categories.

When you move your entire site to a new URL, you'll need to regenerate all counters.

Any direct insertion of data to the database, such as if you're importing from another articles script, will require going through all the regenerations: articles, categories, comments, members, category selector and totals.

Note that regenerating totals is useful if you ever suspect that the number of articles, categories, etc listed on your main page is inaccurate.

Regnerating the category selector / sitemap will update the category selector in the wrapper, on the submit/edit pages, and the site map (that's sitemap.php, not the google sitemaps).