Manual:$wgAllowSchemaUpdates

Details
Enable/disable upgrading via update.php. If this is set to false, the script must be run with   to execute schema updates.

On very large wikis with multiple db servers, updates can cause downtime if done incorrectly and are usually done manually with special processes. This setting is used to prevent people accidentally using the script on wikis where it is not an appropriate way to update MediaWiki (such as Wikipedia).