Not to mention that the entire point of the Xbox division is to get people locked into a ecosystem. It's to try to get you to buy everything, games, music, movies, etc.. through their portal. So it makes a tremendous amount of sense to demonstrate to people that their digital purchases will transcend generations and still be available.
This is the whole point of W10 and XB1 integration. To make sure that everybody moves "to the cloud" and moves to "digital purchases" through a store that they reap the commissions for.
MS providing BC is just another way of saying "Trust us, we'll make sure you can still play your games." And although it was difficult in the past, and still a bit difficult now with the 360-XB1 transition, everything going forward is going to be software based and there's no technological reason why all of these games won't be available to you on the XB4.
And of course, the XB4 probably won't even exist, it'll all just be a software based emulator that is accessed through an app on any agnostic hardware device.
If you look at it in that perspective, you get it. Again, Sony isn't doing this. Sony simply doesn't even have the ability to attempt to do what MS is trying to do because Sony doesn't have a gazillion PC's out there as their market base. Sony has to continue to rely on hardware sales to stay in the gaming business. MS, on the other hand, wants to do everything they can to turn gaming into a software business so that the hardware is essentially meaningless.
Creating BC is a big step in making that point.