If you have very critical data on the drive (work computer, trade secrets, tax information, client lists) then you probably don't want to trust a full format for making data unrecoverable.
If you full formatted and then realized you didn't have some things backed up, you might be able to recover some of it or all. You can probably pay thousands of dollars to some company that will get it back.
There are secure wipe programs for a reason... and different security levels at that...