Because as 0_o Said, this is for maps that Haven't been submitted, and therefore don't get online backup.
Xanandra wrote:Or peppy could make it so osu! keeps saved all versions of the map in another folder, and in edit mode there would be an option to load the diff to a previous version.
I'm saying this because I find myself uploading the maps and asking some people about what they think about a specific pattern very often, and if they don't like it, I have a hard time to reverse it back in case I forget to make a copy of it, and having osu! auto saving all records of them and keep them organized would help a lot. Anyways, it's not like these files were to weight that much....