Can I do anything useful with the cases my PC games came in after redeeming the code?

Solution 1:

If you have enough proof of ownership and the license key isn't printed on the case directly they can be disposed of. But seriously... don't do that, unless you're really low on space.

They don't serve any purpose other than being collected or misused for something else. And actually I buy games with a case on purpose, even if there's only a key inside without a physical copy of the game. Like many others I like to collect my games and looking at the collection of the last three decades is a great feeling.

The value of an empty case on the 2nd hand market is very low, unless you have something really rare. Misusing them is actually worth more. Build a wall out of them or paint your walls gaming-style using cases as wallpaper... that's something special at least.

Solution 2:

Throwing them out opens up the risk that someone will take them out, clean them, and then put them on sale, with "code already used" as a really, really fine print. I have seen auctions like this when I was looking to buy Mass Effect 3. I didn't buy, but someone might - you do not want that for your fellow players. So if you need to throw them out, please, destroy them.

Other option I have seen was to use such cases as back-up for game files. There is no guarantee that the game you bought with lifetime access will still be available for download five years from now. Company might go bankrupt, it may be taken down due to court case etc. Having some way to play what you paid for in such situation seems like a good idea.