It seems to contain the same as the Last Post column. It does not really matter to me, as I am on a wide screen PC. But people on narrower devices (as your iPad in the other topic) may find this waste of space stupid.
Modify date is affected by any moderation change and a user edit of a message - so it's usually the same as last post date but not always. The sort orders and RSS feeds need both columns as they see 'new content' differently.