Enable this option if you want to store your previews in the database. Storing previews will lead to a larger database and the thumbnail building process will also slow down because the previews will also be created at that point.
The positive side is that IDimager can use your previews if needed. For instance; you can carry your database with you and leave the images at home or at the office. Then when you're working offline the previews will be used and allow you to continue working, even when the images are not online.
Using stored previews will also drastically improve the display time of the Preview panel.