It was originally copied from SBTs template for sbt.boot.properties, for which the syntax was that ${{property}} gets substituted when SBT itself is packaged for distribution.  So, when SBT distributes this file, it ends up with a hard coded version number in it.  Since for us, this is not a template but the actual file that we use, that meant we had a bug, which you may have noticed, created a project without a build.properties and run the play command in it, and you get an error saying that version ${{sbt.version}} of SBT can't be found.

I actually want to get rid of this file altogether out of Play, since it complicates things, and makes it harder for users from using their own custom SBT environments.  We do need to address the issue of having so many hard coded SBT version numbers throughout the Play project, and this file, whether it exists at all, and if it does exist how it's generated, will be a key part of that solution, but I think this is a separate issue to this upgrade.  Here I was just fixing a bug.