Why a DBA

After long time I found the best answer from Toad World blog.

Developers and other technicians will bounded for some area. But the DBAs manage hundreds of SQL Server instances, making sure that servers are always online, backed up, and corruption-free. They manage clustering, SAN storage and security. 

Windows admins are responsible for making sure the hardware, storage and operating system works correctly, and the SQL Server DBAs only start troubleshooting at the SQL Server level. If the SQL Server can't access its drives, or if the operating system throws errors before SQL Server starts, then the DBA team calls in the Windows administrators and makes them do the troubleshooting first. The production DBAs still have to understand the basics of Windows and storage, but they're less involved in the day-to-day hardware and OS maintenance.

Production DBAs make sure that SQL Servers are installed correctly, secured, backed up, and when necessary, restored. Production DBAs spend a lot of time in meetings because they frequently help to implement third party applications. Project managers or business managers will purchase applications from vendors, and these apps store data in SQL Server. The production DBA helps define what kind of database server is needed, or what existing SQL Server will be used. The application has its own database schema that can't be modified, and the production DBA may not have to learn that schema, but the DBA is still accountable for making sure that the server is always running, reliable, and fast. When questions about performance arise, the production DBA has to determine whether it's a hardware problem, a SQL Server configuration problem, an application design problem or a misbehaving user.