Defragmentation

Defragmenting brings the information together on your hard drive that should be together but was separated due to normal operation.  This helps your operating system (eg. Windows etc)  gather the required information it requires as the internal mechanical parts of the hard drive don't have to travel as far, thus it should be faster.

Due to various reasons it is not possible to defragment every single piece of information on your hard drive but by performing this task on the bulk of the data obvious speed improvements can be observed in most cases.