andreasaspenberg2: there is external disc drives. i still have a disc drive myself but, the official release is not compatible with my operating system. the gog version included patches files in the dx folder that solves those issues.
Almost all of the fixes are taken from the community, you could do the same with your original disk version. The ddraw.dll wrapper that GOG provides is one they made them self, but there are plenty of community alternatives so unless you prefer theirs there is no real difference.
andreasaspenberg2: on the other hand: you could just buy the game through battle.net.
The one you get from battlet.net is missing most of the fixes and has no advantages over the GOG release, in fact you might not be able to run the game from the launcher there.
If you use DevilutionX it doesn't matter which version you have since it just reads the game data which is the same in all cases (including the Mac release).