A few years ago when I got my iPod, I decided not to import all of the random mp3s that I had from college - mainly because they came from various sources and didn't have all the ID3 tags in a consistent format.
So I started by importing the CDs that I owned, and slowly grew my collection.
Eventually, I moved all those mp3s off onto a backup drive and forgot about them.
Then, the other night, I remember that I had a live recording of a U2 song - and I hadn't heard it in ages. So I found that stash of mp3s and started importing a lot of that music into iTunes; fixing up the ID3 tags as I went.
And now I have all this great music from when I was in college and high school and it's bringing back wonderful memories. What a gift.