what is the use of the migrate module UI in drupal 7
The UI just acts as a display for the mappings that you create, it also tells you how many nodes have been imported and how man haven't so you can manually do it.

There is lots of documentation on the drupal website here http://drupal.org/migrate and the example migrations are worth a look.

To migrate from drupal 6 to 7 I used created an xml view in drupal 6 and then created my mappings to use that in Drupal 7.

Cheers