Creating animations for the web is often a time consuming task. Creatives are forced to learn code and the paradigm of browser rendering.
Obviously, there is nothing wrong with learning code in general, quite the contrary. I strongly encourage everybody to learn it, at least the basics. However, writing large chuncks of code for a linear animation tends to distract you from your mission, which is creating animations.
When I write code for animations, I often find myself in the process of trail and error. Switching back and forth between my code and the browser. This just doesn't feel right.
With realtime feedback, you can finally focus on the esthetics of the animation (timing, etc).
Because animations take so much time to create, it's often phased out, which is really a shame. Besides, web animations cannot be considered as a gimmic or nice-to-have anymore, it's part of a greater good.
Go beyond everyday transitions and animations. Spirit gives you the tools you need to visually animate the crap out of the web.