Because the Install DVD will check your Hard Disk for 10.5 or 10.6, much like the 10.5 Drop-in DVD does.
If you have 10.5 installed, and you boot from the 10.6 DVD, it sees you're running 10.5 and gives you the normal options: Upgrade, Archive or Erase.
If you have 10.6 installed, and you boot from the 10.6 DVD, it sees you're running 10.6 and gives you two options: Archive or Erase.
What if you've got to wipe your drive?
Am I the only one who thinks the new QuickTime icon is horrible?
No, I dislike it as well. It would look good if they changed to current one from blue to black.