I think Samsung originally developed it as a novelty to catch your attention, something out of the norm.
As
@notforhire stated, navigation ease. It replaces swiping in certain places.
I am the worst at swiping on my watch. I seem to always overshoot where I want to be. It's easier for me to just turn the bezel.
You can also use it to:
• Answer or dismiss a phone call. Dismiss a reminder.
• Increase or decrease brightness or volume.
Those are the features that I use.
So it's just personal preference. You don't really need it, but you might like using it.