The table column "term_group" is already part of the wp core but not used till now.
Creating term groups would help to create hierarchical taxonomy structures and better tagging. It could also help creating tag clouds for each group if those functions are implemented.
Examples of tag groups:
* to unite tags per language in a multilanguage blog and to show tag clouds for each language
* to use tags with more than one meaning - e.g. "plane": a flying object, a canvas or a layer - in the right context.