Summarizing the comment here, the reason the Developer Tools need to be installed comes from the way the biber unpacks itself on first run. Each new version of the binary checks to see whether it is different from the previous version. But since Mac binaries support two different chip architectures (Intel and Apple) the binary distributed is a Universal (or "fat") binary, i.e. it contains binaries for each architecture.
The command line tool lipo is an Apple utility which creates or operates on Universal binaries, and does various things. One of those things is that it can extract one of the two binaries included in the universal binary to create a "thin" binary. This is what the Mac version of biber does behind the scenes. It checks which architecture it is on and then extracts the relevant binary for that architecture using the lipo command.
Since lipo is not part of the standard OS, but comes packaged with the developer command line tools, that's why they need to be installed to keep your biber binary up to date.