You don't have to do that. If you're writing against Linux specifically, and your use case could make great use of Linux-specific features, use them! I've certainly done that in the past.

But I've also written a whole lot of software which really couldn't care less about the OS. It needs some way to list directories, some way to read and write files, maybe some way to communicate over the network, but everything interesting happens in code that's not related to interacting with the OS. In those cases, I love that I can just write against one API and know that, if it doesn't already just work on all posixy systems, it's at least going to be easy to port.