Fragmentation

File fragmentation occurs when files can no longer be placed next to each other or contiguously on a disk. Whey you purchase a PC, usually it contains an operating system and some additional files loaded at the factory. These files are mostly loaded one at a time and are fairly contiguous. Over time as you install and uninstall programs and data files, blank spaces appear between files that can be used for storage operations or new programs that you install. If these files are not exactly the same size as the free space, which most are not, the system places them in spaces anywhere on the disk that there is room. This means that files can be spread all over the disk slowing the system down because the seek time increases. This used to be called checker boarding, but I haven't heard that term used for a hundred years.
Since our hard drives have increased in size every 6 hours, the amount of fragmentation has also increased. Data files and program files can be scattered literally from one end of the disk to the other. Defragmenting the drive is an absolute necessity especially when your storage capacity increases. Just because you have more room doesn't mean your system will continue to run quickly forever. It just means you have more room for fragmented files.
Check your drive(s) frequently to see how much space is being underutilized. This can be done in the System or Administrator Tools menus. Select Disk Defragment and click on the analyze button. This will tell you is you need to defragment your drive or not. If you see mostly red lines, you have a mostly fragmented drive.