Ever since we got variable heights at map borders, the outside graphic is not shown anywhere anymore. Yet if you remove the image, the game breaks.
It seems really strange to require an image that won't be seen anywhere, so I guess it counts as a bug. Not a big problem though, just worth mentioning.
I think it is because it is the only plain image the game was guaranteed to have in regular tile size, so it was used for determining tile size (and store name/copyright). Its name is now legacy, but its purpose as an image of correct size and some metadata remains.
Yes, indeed, it server as provider of these two funtions. The pak set revision is optional, but the tile size of the pak set is essential. Thus it is assumed to be always there.