Answer:
When you empty the recycling bin, the computer marks the header of each of those file you deleted as not needed anymore. Therefore if your computer ever needed to use the memory location where those files was once stored, it's gonna look at the file header and be like, "hey, this file is marked as unneeded, therefore I'm allow to overwrite this area now."
If you don't empty the recycle bin then those file can't be overwritten.
