I bought a lot of spare parts over the years. Some of these I needed to refurbish watches, and some I bought as "insurance" against potential damage. In many cases, I lost interest in the watch itself before I ever ended up needing the spare parts. A couple of times, having the parts saved me after a scrape or a scratch.
These watches are not always cheap. In some cases, they are also limited and hard to find parts for after the fact. There are no 20th anniversary parts anymore to be found, and 25th anniversary parts are getting harder and harder to find, with a few exceptions (Casio made a LOT of dawn black frogman resin)
Here are my spare casebacks, which serve a couple of purposes (replace if the original gets scratched up, or replace if the sound stops working due to a bad speaker (it is adhered to the inside of the caseback))
I also have a couple of spare crystals, because you never know when these might get scratched, and they are a pain to polish.
Of course, there is always the mentality of "it's a G-shock, wear it hard and abuse it". While I do think this applies to some models, there are some that I don't want to beat up. I generally am not hard on my watches.
The motivation for buying spare parts has always been "what if you can't get it when you need it?" Because Casio is completely unpredictable in their choices to discontinue or keep producing spare parts, you never know if something will be around when you need it. The parts are usually cheap (casebacks are around $10 for uncoated, $20 for black ion plated, and $50 for DLC). Crystals are like $10. You never know when you will be able to buy a trashed watch cheap on ebay and replaced the damaged parts. I was able to buy and restore a GW-200TC frogman only because I happened to have a spare case center on hand.
Here are some of my parts, mainly for squares. I have lots of other parts, mostly bezels and bands, but a few gaskets and screws and other odds and ends. I know there are some people here who stockpile rare resins, but I am hesitant to have too much resin on hand because you never know what the shelf life will be, especially for resin that is already 10 years old. The casebacks and crystals, on the other hand, you can have for ages.