Alfred's System Commands are so simple to use, they can easily go unmentioned.
From emptying your Mac's trash and launching your screen saver, to hiding or quitting apps and ejecting volumes, the System Commands are brilliantly useful. No need to take your fingers off the keyboard, or hunt for items in the menu bar.
Many of the commands can be set to require confirmation before performing them, so that you don't accidentally restart your Mac when you meant to open Reminders.
Want to lock your screen before leaving your Mac?
My most-used command is the screen saver; My Mac is set to require a password after sleep or screen saver (which you can set in your System Preferences > Security & Privacy > General), so when I step away from my Mac, I can type "scr" and know my Mac is locked.
I've also created a workflow so that I can trigger the screen saver command with a hotkey combo, which makes it even quicker to set!