Disk De-fragmentation is one of the best one to have your files accessing faster. When you copy, paste or change it from drive to drive and delete files in hard disk volumes, there are number of fragmented files on your hard disk and slows down the access of your other files on disk.
When there are more fragmented files on disk we cannot copy or perform any function. The computer effects with the screen freeze, you can notice that sometimes. If your screen freezes often then its due to the fragmented files on disk.
This disk defragmentation keeps your files in order, it piles up all the files so that to get a easy access of the file and copies it faster or while installing a program it installs fast. Such operations get faster if you have a regular defragmentation of hard disk once in a month. Its just simple to do that just start the process of defragmentation and leave your computer for sometime(Time depends on your hard disk space). You can even schedule the scan time for a regular interval of time so that it runs automatically without your caution, it is done by the Task Scheduler.
As far as my experience, the Windows gives a basic De-fragmentation tool which can work well for a normal usage. But who needs to work extraordinary and experimenting with the software then have to use some other disk defragmenters such as Auslogics disk defrag, Diskeeper or O&O defrag.
The other method to maintain your speed of accessing is to use a option called disk clean up. It cleans every such temporary files, dump, windows error files and cookies. Cleaning such things can make your system faster. Cleaning cookies regularly can make you surf your browser faster.
I can say this is one of the "Go Green", when you need a file to access(huge size) or to transfer or to search a file the operation will be performed faster. When system contains fragmented files in hard disk it takes much time for the file to perform the above actions, which ultimately lead to high power consumption and loss of time, So De-fragmenting files make us "Go green".
Perform disk de-fragmentation once in a month or according to your usage.