Partly because I can see what I'm grouping together well, partly because XnView is far more robust than Windoze(Vista) Explorer to renaming the directories.
It would really help in XnView showed the size of the currently selected directory in the status bar, rather than having to keep on right-clicking and selecting properties.
Of course, if there's a way to do that already that has passed me by, please do tell me I'm a fool, and how to do it

Even better would be an "archive media ruler" with advice how the selected directories/files matched standard CD & DVD archive chunks (how much over/under) - but I'm probably asking rather more, there.