I am going to try my best to explain "the life of a HDD," simplified.
When brand new and unformatted it is entirely blank for all practical intents and purposes.
Once you partition it and format the partitions each partition is set up with a file system that allows individual files and folders to be written to it. It is "effectively blank" until you actually do write files and folders to it.
Files/folders are nothing more than a collection of data blocks that form a chain. Each block points to the next block in the chain until you reach the last block of the file, which then points to nothing. Every one of those blocks, for a single file, has something in it.
For that single file, if you delete it, you do not erase the data, all that happens is that a marker on each block that notes whether the block is free for use gets set to the free state. All of the blocks contain the data from your file until the individual blocks are grabbed in order to store something else. This is why undelete utilities can undelete. They can find a series of blocks marked as free, but where all the blocks of the file are still unused by anything else, and reconstruct the file by grabbing all those blocks and either transferring them to another drive with each marked as "not free" or by marking all of them as "not free". They only do this when they can find each and every block for a file (and the number of total blocks is stored as part of the data, so if they get part way through and hit a "dead end" because a block was reallocated they just can't recover that file).
What you are doing when you overwrite (not erase, though a complete erase/zero-fill is the most thorough way to obliterate what was there) using a drive wiper is just to travel, block by block, on the drive and scramble some number of bits/bytes per block. When you do that, you are intentionally corrupting whatever was in each block. Even if a file undelete/recovery utility is used, what it will put back together will have part of the data in each and every block scrambled, resulting in the reconstructed file being unreadable (or at least that's the concept, and the concept works in practice).
Drive Wipers generally have the option to wipe only the free space, or the entire drive. Were you trying to donate a computer or give it to someone else, you generally don't want to wipe the whole drive, just to obliterate "your stuff." One can do this by setting up a new user account with admin privileges, deleting all other existing accounts, then running a drive wiper on all the free space on the drive. You don't want to wipe out the OS or the brand new user account and its files, only the stuff that was once used for other user's files that's now marked as free, but still contains the data necessary to undelete files unless you overwrite it.
Edited by britechguy, 28 February 2018 - 11:47 AM.
Added final bit about drive wiper software and how its used