in short. and i mean very short as im off out now... every time you delete or install something on your HDD it will produce holes/gaps between your files making them open a lot slower. windows built in defragger is rubbish. if you can afford 'perfect disk' then use that. i do and my hdd only needs defragging once a year ish, if that. defragging puts stress on your hdd in order to do its job so you dont want to be using it more than a few times a year.
Your computer may just begin to run a bit faster than it used to, mine does. It's important to clean it every other month to keep your system safe, and clean.
Defraging (can't spell that) will help the PC instead of hurting it like BIG programs do.
so all it does really is fill the gaps
As well as what Matt said.