Please pardon my utter ignorance, but could someone explain to me what a "defrag" is & why it's good for my computer?
Montana's explanation is correct. Here is more detail.
Think of the harddrive like a CD. You save a file, it gets saved to track 1. But it does not fill track one, so you have half a track available (I know CD tracks don;t work this way but bear with me).
so the next file you save might need two tracks, it will take the half track on track one, but it also then saves to the next available trackj (maybe track 27).
So when that file is read, it will first read a bit of it and then have to move somewhere elese to get the rest.
The file has been separated into fragments.
To defragment just makes the files contiguous.
An easier way would be to deny all cookies and specify exceptions to the rule. That way there is no need for the daily check.