Let's say you have 500 tags in your portal and want to merge two tags. At the moment you would go to control panel on the tags administration, mark both tags and click on actions>merge. However, this is almost impossible if both tags are on different pages. If you mark the first tag, select another page and mark the second tag, the first one is automatically unmarked.
To solve this, Liferay have to preserve all marked tags on all pages.