It is not uncommon to see German folks (usually men) wearing sandals with socks. In fact, it's even possible to receive, as a gift with purchase of sandals, special socks to wear with sandals (as I wrote about here). I think it looks really, really silly. After all, if one's feet are cold enough to need socks, then why not just wear regular shoes?
I actually was able to speak to a real, live German* and he shared his thoughts about this. He thinks that wearing socks with sandals makes sense because one can air his feet out yet keep them warm too.** So, there you have it: some reasoning from a German male regarding this practice.
*I am being silly here, in case that wasn't obvious.
**I teased him and told him that wasn't a good enough reason and it still looks goofy.