Sometimes you forget the name of a command but you know what it does. This command looks through all the help files and finds potentially relevant help for you.
Honestly, I only use this out of desperation. I first go search online and can usually find a better page describing what I want to do. If I still can't figure out what the name of that command was then I go through this help list until I see it.
For the price of most other course's PDFs only, you can get the full PDF for this class and 2 videos demonstrating the whole book for both Unix/OSX Terminal and Windows PowerShell. The course is self-paced so you can go through it any time you want, as many times as you want.