I think you mean, "Why" is it not a good practice to save everything on the desktop.
There is nothing wrong in saving everything on your desktop but you should create Folders and organise all your files properly. For example, if it's something related to school, you can make a school folder and save all your school stuff on that or if you downloaded a game, you should create a games folder and then save every game you download on that.
If you have too many folders in the desktop, you can just move some to the Documents.
If you save everything without a folder on the desktop, then your desktop would just be a huge mess, and it would be very hard to find anything on it.
