Just wondering what would the constant be good for? When would you want something instantly snapping from one spot to the next? Well, maybe eyes but what else.
Constant is great for blocking and things that need to turn on/off like a switch.
You can also use constant if you want to animate on 2s (hold every pose for 2 frames)
It is way more useful than you first think. But you will learn more about blocking in the next animation course ;)