This column is used to determine whether a page is "new", meaning:
- It has a single revision, or
- It's just been undeleted without additional edits
The second part doesn't seem to make much sense, and talking with @daniel we agreed that it looks more like a bug than a feature. Fixing this not only requires a code change, but also a maintenance script to set page_is_new back to 0 for pages with more than one revision.