InDesign files can quickly end up very large, taking up valuable processor and RAM, not to mention filling up your hard drive, server or Dropbox space.
Image previews, deleted pages, etc all remain in the INDD file until you take specific steps to do something about it.
Choosing “Save As” every hour or so and overwriting the original document is a great way of resetting the file. Typically a document will reduce to 1/4 of its previous size.