On the Android Enthusiasts SE we get a fair number of Android development questions which are, invariably, migrated to Stack Overflow.
However, the tags that might have been appropriate at AE are not the same as for SO. Let me offer a concrete example: I just migrated a question tagged "2.3-gingerbread" and "development". On AE those tags make sense, as that's the OS being discussed and the topic. However, on SO the question should be tagged "android" and, perhaps, "touch-screen".
Now, I could change the tags on AE, migrate, then change them back. Trouble is "android" is on the tag blacklist for AE. Or, I could edit the tags on the new question on SO, but then I'm not a mod on SO. Either way it's a fair bit of trouble to migrate a question and make it as useful as possible on the destination site.
So, it would be nice if the migration tool offered a way to modify tags as part of the process of moving the question.