It doesn't apply now since it hasn't done it, but remembering several years back when Offline Mode refused to work on Steam at any time, it would become an issue.
Plus there's the fact that a slow Internet connection = slow install time when transferring from the disk itself could / would be much faster, not everyone has the luxury of running high speeds at a constant rate.
And having the disk simply becomes irrelevant anyway once it's added to the [Insert host here] library, there's no point for games to even be shelved anymore with items inside the box except maybe game manuals, they're there for the sake of advertising to people who aren't at their home buying the game from Steam itself.
