- For minimizing a window from the launcher. Right now you can restore them but not minimize them, so the process lacks symmetry. Of course you can minimize it from the titlebar buttons, but users are used to being able to minimize windows from the task list so that's a choice you're taking from them.
- For going straight from an app to a specific window from another app, without having to (first) go through that app's most recent window, (second) open expose and (third) choosing the desired window. Currently the result is the same (you get to the window you wanted) but it forces you through more steps and, visually, can get more tiresome (windows zooming in and out every time you want to reach one of them).
- For keeping track of an app's open windows while you're working on another, without having to lose focus. Also for minimizing / closing an inactive app without having to focus it (e.g. if you want it to remain hidden because the boss is coming).
- For opening a new window straight from the launcher. You can do it with the first window, so why not with a second one?
- For dragging contents from a window to another. For example, you want to drag a file from Nautilus window A to Nautilus window B, but B is hidden behind other windows. You would be able to drag the file to the Nautilus launcher icon, then to window B (which would be brought to the front) and then drop the file. The task list has this functionality, so that's yet another thing that Unity is taking from the user.
All in all, it's a handier way to keep a track of windows, switch windows and perform operations on windows. The user can do all those things now, one way or the other (even the one about opening an additional window, because it can be done from the window's file menu, and the drag-and-drop because you can bring the window to the front in advance); but the process is always longer and more confusing that it would if my proposal were implemented.
The same problem exists with the current implementation, because how do you check the name for a launcher icon on touch screens? But it could be easily solved by implementing a second way to launching it: by keeping the launcher icon pressed for a moment, without dragging it in any direction.
- If you hover the mouse over the launcher, the window spread appears but if you move the mouse away it disappears.
- If you press the launcher for a while it also appears, but doesn't disappear until you click somewhere else out of the spread.
Consider that a user without a touch screen wouldn't even notice that the second method exists, since the spread would be there as soon as he hovered his cursor over the icon, without having to keep it pressed.