Your hard drive is a neat and tidy place to store all of your data. And if you believe that, I have ocean-front property in Arizona you can buy. You may keep your files well organized. But the way your hard drive stores data isn't always so precise.
Related data is often fragmented across different sectors of your drive. Half of a program or file may be in one place. The other half is somewhere else. The program or file will still work fine. But accessing it won't be as fast as it could be.
Defragmenting reorders the data on your hard drive. It's an attempt to pull related data together. Windows has a built in defragmenter. But using it can take up a lot of your processing power. Auslogics Disk Defrag Screen Saver is a better way.
It replaces your screen saver. It automatically starts when you're not using your computer. When you come back, it suspends itself. It's never using processing power you'd like to use elsewhere. And when you go idle again, it picks up where it left off.