The correct answer is almost never.
Please see the below knowledge base article for alternate ways to push changes through on your site:
Why does an old image or main header image keep appearing when I try to update it with a new image?
The only time we find it necessary is very occasionally a clump gets "stuck" when articles are removed from it but they are still showing.
Sometimes when you see a situation like that, a site rebuild will resolve it. That's because it goes through and manually republishes every article and department on your site.
Please note: It does not overwrite any custom files that you may have created and placed on your site.
Your Site Rebuild may time-out and need to be restarted. If this happens click the "rerun site rebuild" link shown below (this may need to be clicked more than once):
Once the rebuild has completed successfully, you will receive a successful completion message: