So its contents aren't lost if Earth's surface gets depopulated.
I like the idea of periodic Wikipedia moonshots, although the storage format is kind of an open question, I've wondered for awhile if a DVD made from e.g. quartz, platinum, and titanium might be up to the job.
A full backup would fit on 12 double-layer, single-sided disks; I'm being conservative and not using Blu-Ray numbers, since density and longevity are always somewhat in tension. Probably more expensive to put them safely on the moon than to manufacture in the first place.
In space you probably don't have to worry as much about minor surface scratches and oxidation, though. You just have to worry about outgassing and meteoroid impacts. Some of them you can stop, and some you can't. On the bright side, they're very rare.
I think common media formats like DVDs are designed with a lot of emphasis on speed, both of reading and of duplication. This compromises both density and longevity. If you, instead, allow yourself the luxury of FIB milling to write and an electron microscope to read, you can manufacture in any vacuum-stable material at all, and you can engrave your archival message with, say, 50-nanometer resolution. At one bit per 50 nanometers square, you get five gigabytes per square centimeter.
I think that with e-beam photoresist cross-linking followed by etching you get about 500 kilobits per second, and I think FIB milling is a little slower, so it might take a few weeks to make the copy — obviously unacceptable for a consumer CD burner but fine for periodic Wikipedia moonshots.
There are 14 people on the ISS. If they were the only ones left, they would certainly not have the breadth knowledge of a Wikipedia dump.