Apart from reducing OS bingo (no need to deploy Windows to run SQL Server alongside whatever non-Windows OS you use to run your apps on), what is the selling point?
I assume it runs on x86 processors. If I do everything wrong and end up needing a ridiculously powerful machine I can check IBM's POWER-based boxes or Oracle's SPARC machines that run MySQL or PostgreSQL (or Mongo or anything else) and scale vertically well beyond the largest Xeon boxes. But those won't run SQL Server. Being that the case, why not Windows?
Windows licenses cost. Also if you are running your other stuff completely on Linux, having to setup Windows boxes just for the SQL Server can be bit inconvenient. I see this as a companion to the multiplatform .NET. Now you can run a Microsoft shop without running Windows.
I assume it runs on x86 processors. If I do everything wrong and end up needing a ridiculously powerful machine I can check IBM's POWER-based boxes or Oracle's SPARC machines that run MySQL or PostgreSQL (or Mongo or anything else) and scale vertically well beyond the largest Xeon boxes. But those won't run SQL Server. Being that the case, why not Windows?