Probably the app I use the most without ever noticing. I recently changed computer and the 10 minutes I spent before installing it again seemed slow and tedious.
You quickly get accustomed to using it to search anything on your computer, whether it's launching an app (already open or not), accessing a file, searching on Maps or Google from anywhere. The beauty is it that just typing in the first 2-3 letters, it always shows me what I want.
Simply put: Alfred is to Spotlight what Google is to Bing.