In the last century, more precisely in 1996, security researcher Peter Gutmann published a study claiming that overwriting data on a magnetic disk would leave magnetization remnants of the overwritten data at the borders of the track which, given sufficient resources (say, a state actor) might allow to recover part or all of the previous content. Elaborate strategies of repeatedly overwriting with various bit patterns were designed to minimize the chances of such a recovery.
With modern magnetic storage methods and densities that problem does not exist anymore, and with SSDs it never existed. The various recommendations to overwrite deleted data more than once (such as the famous DoD 5220.22-M 3-pass standard) are therefore obsolete and have mostly been revoked. But the story persists in collective memory and as a consequence, programmers worldwide reimplement that standard to this day because "it is known" that to erase data reliably "you have to" overwrite it multiple times.
Modern storage media present very different challenges for making sure deleted data is unrecoverably erased. Bad block replacement, caching, wear leveling may all result in data persisting somewhere on the device even though it has been deleted or overwritten. Multiple overwrite passes do not help against these.
In sum, overwriting more than once does not buy you anything on a storage medium that's less than 20 years old, except perhaps on floppy disks if you still have them.
