Choice
Debian, as with many things Linux, gives you the choice. You can install from ISOs, you can do the pseudo image thing (though I've heard quite a few horror stories from same), or you can install from a base image plus floppies. Point being: there's more than one way to do it. Frankly, more choices sounds better, not worse, to me.
Where I've got a network connection, I far prefer to do the minimal (base/floppies) installation, followed up by an over-the-net update. Even with a slow link, this tends to work well. If needs be, yes, ISOs can be used. They're by no means necessary.
The same can't be said for most other distros, AFAIK.
