Actually, I think HeR does it so that the people who ordered the physical version of the game with 3 to 5 business days shipping, have a chance to get their game by Tuesday - that's when the digital version releases.
I agree - that's another equally good reason for shipping out early.
And it makes good business sense in general to do it this way. I can remember instances of folks who had preordered disk versions of games who received them after the game had been released in retail stores - and that rather defeats the purpose of preordering. People who had preordered the game still did not have it, while others were already playing it.