There is lots and lots of hosting companies that ask you for a couple of dollars and give you your own machine for that.
This is normally the cheapest way to have your “own” server in the internet.
After a couple of years, you realize that there is a better deal including better hardware at some other hoster or even the same.
So you decide to move your stuff over.
Now you want to cancel the current contract, but what is happening to the data that is on that server. Simply deleting is not really a solution. There are plenty of reports of harddisks turning up on ebay and the new owner has no problem restoring your data.
There is a solution to that called shred. It is a basic tool that is available with all popular linux distributions.
The easiest way is to use the following command. shred /dev/sda -f -v -z
This will overwrite your harddisk 25 times with garbage and it is really, really hard to get your data back.
Here is the shred man page.