Backups and restores.
Oh, those weren't on the list. I'm going to support Gail's answer. If you spend more time building and designing systems, having a working knowledge of how the app code is written is more important. If you spend time doing installs & system configurations, then understanding Windows is more important.
"The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood"
- Theodore Roosevelt
Author of:
SQL Server Execution Plans
SQL Server Query Performance Tuning