OK Fishu, You asked:
Why reformat the hard drive? I've never reformatted one, other than the brand new drives (not much of a choice there).
I gave your four specific examples of when you would need to reformat a drive. And even though, by your own admission you have no experience with it, you proceeded to argue my points, and quite poorly.
My first reaction was that you obviously have no idea what you are talking about. For example, you can't change the allocation of space on a hard drive without disrupting the file system. Therefore, if you repartition your drive you have to format the new partitions with a new file system before you can access them with an OS. This should be quite obvious. In fact it's so obvious that I considered that you might be trolling. Either you're trolling, or you like posting arguments about things you really don't understand.
Then I saw you are from Finland, and considered that English is probably not your primary language. Maybe you misunderstand the "formatting" we're talking about. Since you are probably an intelligent person, I gave you the benefit of the doubt.
So, which is it? Do you not understand what we're talking about, or do you just argue things you don't understand?
If you really can't think of a reason why someone would want to format a drive on occasion, then you really don't have any credibility with me in this thread.