Add another is designed to save time during repetitive content creation. It allows the content creator to add another node of the same type much faster. The user interface modifications it provides to achieve this include:
- Add another message displayed after the user creates a node
- Add another tab on nodes to create further nodes of the same type
- Save and Add another button on node creation forms to allow the user to quickly create many nodes of the same type (Drupal 7.x-2.x+ only)
Drupal 7 moves the Add another settings onto the node type settings pages to allow finer control. Previous versions supplied a single configuration page to administer all node types.
Add another is an attempt to check one of the items (#5 on my list, actually) off the wish list on my blog. The first version was created in about 30min.
Add another 2.x and higher include all the functionality of Submit Again and many more configurable options, so it is unnecessary to use both modules. In version 1.x and before both Submit Again and Add another may be used together without issue.