From a business point of view, this make great sense for Marvel.
They have this huge backlist of comics, decades worth, that are just sitting around not making money. They could reprint them in trades, but printing is expensive, so by putting them online they can end up making a lot of money for very little initial cost (because web hosting is dirt cheap compared to printing).
From the business point of view, I can't believe they haven't done this before. It's basically the same thing that TV studios have realized with DVDs and online distribution. They have a bunch of product that they paid millions for over time and now they have a way of making money off it after it's initial run.