Once a file is deleted, most operating systems will still hold on to the file. The link between the operating system and the file is removed, but the data is still on the disk waiting to be overwritten or used for something else. A common utility seen in the wild is Eraser but it's a bloated utility that takes a long time. It's a good utility if you really need to overwrite a Windows machine more than 3 times - but the use case for this is minimal.
pause: this article surrounds mostly HDD, as files are recoverable on these drives if not overwritten. For SSDs, this is just going to cause more wear to your drive!
An underutilized tool is cipher. In Windows, it displays or alters the encryption of directories and files on NTFS volumes. But, with the option w it overwrites deleted files and empty space of a drive. You can use it on the same drive the OS is located, external drives, and removable media. It's easy to use! If your OS is installed on C: and you want to remove all your deleted files and empty space try cipher /w:C or replace C with the drive letter.
The process doesn't take long, but it is a good idea to not only close applications it recommends, but to close background processes as well. My favorite shortcut for the Task Manager is Ctrl + Shift + Esc, make sure to close as many background processes as possible while this is running.
The best use case for this is when getting rid of HDDs before reusing them elsewhere for other purposes. In lab setups, this can be quite useful.
Do you overwrite space on your HDDs? Why or why not?
Fun fact! Did you know modern OSs already scatter data around when writing to a SSD to more evenly wear it and reduce malicious recovery? Cool!