Hi Zachary, good question.
Well, like most things, it depends. If you aren’t using many or do not require, then it doesn’t matter too much, however..
I would say that higher quality, performance, and control can be considered reasons you might want to go with CSS.
While it is possible to get past the 256 color restriction and lack of alpha transparency in Gif’s by using animated png’s (apng, not really gif, but close enough), good ole Micro$oft (Both IE and Edge) does not support them. That could be the biggest reason right there.
Additionally, CSS animations like the spinner have almost no file weight since the 99.9% of the code is located in the CSS file. For this reason they also do not require additional server requests, which saves load time.
You might see this argued all over the Internet, CSS animations are smoother and perform better.
You can dynamically animate/change/morph any part of a CSS animation, you can not do that with an animated gif.
You do not need additional software or online generators to create CSS animations (although there are some fun playgounds to explore).