Free space

Windows is a "noisy" OS. It writes a lot of data onto your hard drive without your knowledge. Sensitive information that is stored in memory (eg. passwords, encryption keys, etc) may get written to your drive when there is insufficient RAM. There are also the files that you may have forgotten about after you deleted them (not using secure deletion). The inclusion of wiping free space in CCleaner is a useful feature for those who do not have Eraser installed.
Caution! If you have a large HDD, the more passes you select will result in CCleaner taking longer to wipe.