Because all your Internet activities, like the places you have visited, are logged into index.dat files and can be read by anyone. This software is for you, if you like to keep your browsing habits private.
Besides Complete Internet Cleanup clears Windows Temp directory, Recycle Bin and Recent Documents folder to ensure a problem-free Windows operation.
There are three index.dat files residing in Temporary Internet Files\Content.IE5, Cookies and History\History.IE5 folders. They keep basically a copy of whatever is in that folder and they continue keeping those records even after you have deleted the contents of those folders.
Not as long as Windows is running. That's why Complete Internet Cleanup needs to restart your PC in order to delete index.dat files.
However as soon Windows starts, new empty index.dat files will be created, ready to record your steps.
The size of Cookies and History index.dat files should not be more than 16 KB.
The size of Temporary Internet Files (Cache) index.dat files should not be more than 32 KB.
Windows programs create their temporary files in the Temp folder. But sometimes they fail to delete them after use. An example would be those time Windows stops responding and you push the Restart button on your PC.