Disk Cleanup - why?
Disk cleanup is the tool that allows you to get rid of unneeded files. These files take up valuable disk space and slow down the system. Temporary internet files downloaded when viewing web pages are one example of this. These files are stored in a cache on your hard drive, and without emptying them, will take up space. They're not necessary to run your system,they're just needed for viewing the web page and will be renewed the next time you return to the site.
Scandisk - why?
Scandisk is used to check on the folder and file structures. It combs through your hard drive, making sure all files are not seated on a bad sector of your hard disk, and will attempt a recovery of your files should they be sitting on a bad sector. This is a critical issue as files on bad sectors will generate issues and errors.Disk Defragmenting - why?
Disk defragmenting is fine tuning your computer. What disk defragmenting does is take each file and optimizes the file by placing it in one contiguous file. When you install software the software is placed pretty much like throwing paint against the wall; it will splatter all over and make a nice mess. Disk defragmenting causes the computer to look at the splatter and organize it into one straight file. This makes it easier to locate the file and also makes it faster to load since it is all together.