DAP has its own User database, and doesn’t use the WordPress User database – for many reasons including (but not limited to) the following…

1) Ability to store more user information than what WordPress allows

2) More powerful user search, profile updates, affiliate information, etc.

3) Insulating your member information from potential issues arising out of conflicts or changes in new WordPress releases.

So, if you want to use any WordPress based plugins – like WordPress-based Forums (bbPress or Simple:Press), Subscribe2Blog, or want your logged-in members to comment on your blog posts or pages and have WP recognize them automatically, then you need to set this to "Yes".

Documentation: Wordpress User Sync