Think of your hard drive as having a bunch of small cubbies in it, not one massive storage hole. These cubbies are much smaller than most typical files these days. So when you have a file to save, the computer breaks up that file into the correct number of pieces to fit into those cubbies. It could start out as it just goes from cubbie 1 to 30 for one file, 31 to 40 for the next, and so on.
Now in order to be useful, the computer has a log of all these files with the locations of all of the cubbies to make up the full file. It also has a list of "available" cubbies to put new files in. So when you first start out it's possible to go from cubby 1 to the end. But as you use the computer and make and delete files, these cubbies are "out of order" but the computer keeps track of what cubby is "used" and what is "open". Now when you delete a file, the computer just deletes the entry of the file and the associated cubby locations, moving them to the available list. It doesn't clean out the contents, because well it doesn't need to. So now a new files needs to be saved, it is given a list of cubbies to use and as it moves through them just tosses what was previously left in them and places the new file info in there.