imagine you're copying an entire directory structure from one disk to another using windows explorer. It's like 5 gb, but there turns out not to be enough space - there was only 4.5 gb.
what happens?
1. it doesn't do any checking, just starts copying even if there isn't enough space
2. it copies the files over in a non-obvious or controllable order
3. when it runs out of space, it pops up a warning and kills the copy process. so it's impossible to open another window, make more space, and continue
if you free up more space and start the whole thing over, it pops up warnings "the target location already has directories of the same name. if you proceed files in them will be overwritten". you can say yes, and wait through it overwriting all the files with exact copies of themselves, or you can say no, and it'll ignore that whole directory, whether or not it'd been completely copied before or not.
if you were doing a say 3 hour transfer with say 1000 directories and this happened, you would be absolutely screwed. you'd have to either do the whole thing over, or individually copy over every file. and that takes forever because in explorer it's constantly jumping focus around, since the right pane isn't independent of the left pane.
windows explorer is a piece of shit. i use servant salamander instead.