Physical disks will degrade over time, especially in a dry climate. (I backed up my entire CD collection to my computer when I realised this could happen - I already had a couple of disks from the late 80s that were unplayable.) With a digital copy, you lose access to the game if the company folds, but most of us are well down the road with other games by that time. Even if you do have a good copy of the physical disk, it may not work with later versions of Windows - my Creatures 2 disk is still good, but it won't work with Windows 7 and above, even in retro mode.
In any case, I'm all in favour of reducing my environmental footprint wherever I can, so less stuff is always a good thing (and I certainly don't need more stuff in my house). Plus, not having to shuffle disks all the time is a big plus.
