You can use it to look at what you looked like years ago and wistfully think about how the years haven't been kind to you. It can also be used to prevent tables from wobbling on uneven floors.
It depends on how likely you are to wind up in a different country. My wife recently needed the passport number from the passport she arrived in Ireland with. That was 3 passports back, and I'd very nearly binned them.
If you renew online the edges aren't cut off, unless you do it yourself. Not a bad idea either as I know of someone who brought the expired one instead of the new one.