Because:

1. You just never know when some image that didn't look good enough at the time now looks great...
2. Software improves over time and becomes better at extracting the information in a file. Suddenly you have "new" data to work with...
3. You (hopefully) learn how to use software better and learn new techniques that allow you to make the image you imagined when you took it...
4. You are hopelessly anal retentive...

So I rarely delete a file: even those that are under or over exposed, not what I think I wanted, somewhat out of focus (that now looks good out of focus). It's also why I shoot raw files: raw processing programs are getting significantly better at recovering both highlights and shadows you once thought were not there. They are now.

So case in point is this image taken back in 2011 with my second digital back and my Hasselblad Flexbody camera (sadly now gone/sold). My old habit of keeping every file (not only accessible, but safely backed up in a separate location: yes, that's over 16TB of data...) is only reinforced.

Lighthouse Flight

Light House FlightPigeon Point Lighthouse, San Mateo, CA