Because, while the title is the same, probably it is different... sometimes:
Luckily I have retained all the DVD even when upgraded to BD, and very few times DVD had exactly the same content of the BD...
- DVD has different cut
- DVD has different color grading
- DVD has different aspect ratio
- DVD has different sound mix
- DVD has more audio and/or sub tracks
- DVD has extras while BD has not, or less, or different
Luckily I have retained all the DVD even when upgraded to BD, and very few times DVD had exactly the same content of the BD...