"Oh hey look at all of the FOSS software we are using but not providing back to."
This page is just a central place for Apple to publish the corresponding source code for all of their released software. Among other things, this is a simple way to comply with licenses that require source availability, like the GPL.
"What else has apple given back to the open source community..?"
They released all their work on WebKit (including a whole new JavaScript VM) since forking it from KHTML, and have also done a good job of managing it as a real open source project (i.e. accepting outside bug reports and contributions, and giving commit privileges to people outside of Apple).
They've also made major contributions to LLVM, and released Clang (their LLVM-based compiler for C/C++/ObjC).
"All right... all right... but apart from better sanitation and medicine and education and irrigation and public health and roads and a freshwater system and baths and public order... what have the Romans done for us?"
They've done a few nice things (let's say the baths, but not much else), but their instincts are certainly not friendly towards openness. It's tactical cooperation, kind of like in the saturday morning cartoons where there's always one episode where the bad guy has to cooperate with the good guy for some reason, and it turns out they work well together, and the good guy suggests the bad guy give up his evil ways, but it of course never happens.
Not to mention releasing Darwin, libdispatch (Grand Central), OpenCL, blocks, Bonjour, Calendar and Contacts Server, Darwin Streaming Server, launchd and XQuartz.
Others have provided stuff they have done, so I'll point out that they didn't do CUPS. They started using it in Mac OS X 5 years after it started and 5 years after that hired the developer.
Sure they did do CUPS, but what else has apple given back to the open source community, besides another source mirror?