Defragging will help the computer run programs or fetch data that is spread in fragments all over your hard drive.  All Microsoft updates, for example, will be a different size to their predecessors.  SO if they overwrite, they will put an extra fragment somewhere else on your hard drive.  A regularly used spreadsheet or word document could create a new fragment every time you save it.  So when you defrag, you help the loading of those files because they are all in one place in sequence.

For things like old backups, or files you are never likely to change, you do not need to deffragment them.

Try defraggler, which I have used for a couple of years without trouble. It comes as a free version, and can replace the windows defragmenter.  It is regularly updated.  It has a graphical interface showing what is happening, along with information about how badly fragmented your drive is, and even how many fragments each file has.  YOu can see the fragmentation going down as it runs.  It lets you omit files from being defragged, so you do not waste time defragmenting your old MS Outlook backup file, for example.

I have a 1TB drive for data (lots of photos), and a 300GB drive for programs, and use it on both on a schedule every week.  It keeps things tidy and never takes long to run now, though it did the first time!