It will significantly reduce the licensing implications of using SQL Server for a small project. I know people who use MSSQL in their day job but postgres/other for personal projects and one of the reasons is needing to license Windows if a personal project becomes public and needs hosting. This way a small project can use SQL Server Express on Linux just as cost free as progres (or mysql if you must) on Linux.

The choice of OS will no longer dictates the choice of DB, that decision can be entirely made on the needs of the project and its target users.

Small projects using Express edition may grow into larger ones needed standard/enterprise features, and people exposed to SQL Server through projects using it on Linux are less likely to dismiss ti later because the know postgres/other only. I'm not sure how much we'll see brand new larger projects use SQL Server on Linux any time soon though, but I imagine "get them in at the ground floor and create familiarity as early as possible" is the intention here much like cheap educational licensing for Windows, Office, & full VS and the availability of VS Express.