This is an interesting point, because they are kinda infinite spaces but they also impose a
structure on the space, which I would assert is a part of why they are so successful, in contrast to the "infinite structureless blank paper" that the OP is talking about.
The trick is getting the amount of structure just right, not too much to be too restrictive and not so little that users are lost in the way the person you're replying to describes.
