Whats the function behind this in the database and why is it so large?

The Import Map stores all data that you *imported* from either UBB.Classic6 or UBB.Threads6... It holds the mappings of "what old post" is "what new post" so the importer can process things properly.

If you're not using one of the redirectors from an import from them, you should be able to do away with the import map table's data.

The importer never worked for me from day one so I have never used it.