Vault maintains re-index status for every file. So even though certain criteria are selected for a re-index operation, it is not necessary that all files that fall under the criteria be re-indexed because some files may be up-to-date thus not needing a re-index. However, the Administrator may choose to force a re-index on all files that meet the selected criteria by selecting the Force Re-Index option that can be exposed by expanding the dialog.
Force Re-Index is especially useful when a new iFilter is added, and also after migration. Apart from that there is really no need to run a Force Re-Index.