The first is to remove “malicious” software, in order to allow your machine to work more effectively. This can include spyware, adware, and viruses.
The second is activity history and deleted file cleaning.
1. Malicious software
Spyware and Adware are designed to collect information about you and your computer activity in order to achieve something for the author. This may be that they want to sell this information, or that they want to use it to target advertising in your browser. They can slow down your computer, and inundate you with “pop-up” windows.
Viruses can be more malicious, in that they can actually change information on your computer (or delete things) and they spread automatically.
Removal of these types of program can greatly improve the performance of your computer.
2. Activity History and Deleted Files
Almost everything you do on a computer is meticulously logged, and this information stays available to anyone who knows where to look.
If you throw your machine away with the hard drive as it is, that hard drive can provide someone with your bank details, personal email, photos and so on. Deleted files can be easily recovered even after a drive has been formatted, so it is always a good idea to clean drives before disposal or re-use.
Internet history, recent file lists and similar information can also be effectively removed to a standard accepted by the US government.
(Please see terms and conditions. While information to be deleted will not be examined prior to cleaning, should any evidence of criminal activity be uncovered, it will be reported to the relevant authorities.)