mg2wpforo is part of the migration tool. It is used to track the migration of old topics/posts id to the new topics/posts id used by wpforo.
We kept ours for record keeping for now. Until they have completed the quotes migration part of the plugin. That is the only thing holding up our 100% complete migration from bbpress to wpforo. Once we have that we can safely migrate from bbpress to wpforo. Soon we hope.
You can backup and delete this table. At the moment there is no other tools which may use this table data, If there will be some new tool you can restore it and use. wpForo core doesn't use this table, it's only used during migration process by Go2wpForo tool.
In case you want to say thank you !)
We'd really appreciate and be thankful if you leave a good review on plugin page. This is the best way to say thank you to this project and support team.