Master database is a system database and it contains information about running server’s configuration. When SQL Server is installed, it usually creates master, model, msdb, tempdb, resourcedb and the distribution system database by default.

Only the Master database is the one that is absolutely a must-have database. Without the Master database, the SQL Server cannot be started. This is the reason why it is extremely important to back up the Master database.

To rebuild the Master database, run Setup.exe, verify, and repair a SQL Server instance, and rebuild the system databases. This procedure is most often used to rebuild the master database for a corrupted installation of SQL Server.