For my manuscripts—a book has three completion dates. The first is when the story is written. Next, it needs revised and edited. Last, the process of looking for those typos gets serious.
Designating a date to finish a project creates an opportunity to evaluate how much time it's going to take. It gives a chance to prioritize. Then, mental warning buzzers can start giving out alerts when the date is looming and attention has been diverted to another activity. For instance, looking a photos of deer on the trail cam when a book is due. (See above for example...)
For me, whether it's written in ink and notarized, or just written in my thoughts—once I decide to write a specific book and my fingers go on the keyboard, I plan a realistic completion date.
Deadlines make the story a serious commitment and not just a fuzzy image of something I might finish someday. Setting an end date makes the journey a little more intense from time to time, but it speeds up the arrival to the finished manuscript.