There is no need to use Internet Washer software for Nici, but you may need one if you have just switched to Nici.
Internet Washer software is used to remove all files that a web browser like Internet Explorer leaves lying around. Programs that snoop on your privacy know where to look for these files and can see where, when and what you have been browsing. An Internet Washer simply deletes all these files.
The files can often be useful, but an Internet Washer program will probably delete the useful files as well as the files you do not want to keep so that your privacy is protected.
Nici stores all its files in a proprietary format away from where programs like Internet Explorer stores its files. A snooping program, or person, would not know where to look or what they were looking at.
Nici also has its own Internet Washer feature which is called Wipe then Exit. This allows you to completely remove all the files that Nici stores. While using Wipe then Exit limits the usefulness of some of Nici's extra features, it means that you can finish and start Nici with a clean slate each time.
If you are just starting with Nici, consider using a program like Windows Internet Washer to give your browser a spring cleaning. You will be amazed at what a program like Internet Explorer leaves on your hard drive.