Windows 7 and most modern versions of Windows have a built-in defragmenter tool to remove fragments from the hard drive. A good rule of thumb is that if you have many fragmented files, your computer will slow down because your hard drive is not seeking data efficiently. It turns out the hard drive is the slowest component in a computer, and the slowest sub-activity of any hard drive is seeking. It is the reason why SSD drives are "better"; they do not do any mechanical seeking. When a computer's hard drive is heavily fragmented, it will spend a long time "running around" like if it were a mailman going to random houses in given area instead of going door-to-door.
Now that you are convinced why you should defragment your computer once in a while, you may notice the built-in Windows tool does not have many options. Introduce Defraggler. Why do I prefer Defraggler? Defraggler gives the option to defragment individual files, shows the size of the largest fragmented files and has an interface that shows the progress.
If you do not want a full system defragmentation, you get to pick which files can be defragmented or which ones are not worth defragmenting. Leading to my next point, if you have a large fragmented file, you might just want to delete it if it has little to no use for you. Finally, being able to see the work in progress reassures you the work is getting done.