They can be useful for hiding transitions for things that are slow. However instead of engineering an animation stack, make the backend faster....
cough linkedin, salesforce, workday cough
As an extreme example of UI animation, here's Persona 5: https://youtu.be/XniK5Q5amoA?t=2m44s
>for most people animations are not smooth
I don't know about most, but it's definitely true that there are processing limits inherent to user's hardware that make certain animations run very clunky, thereby turning a perfectly good animation into an 'intrusive animation'. Because we often have no control over the user's hardware we have 2 options, stay conservative and optimize things that improve the UX for everyone or ignore the possibility that certain users will have a bad experience and push forward with such animations.
Unfortunately, many product owners tend to build for only the 'happy path' scenario and choose the latter.
OSX is considered to have good animation flow, but its still too slow. Any animation that is inserted between fast moving actions is an obstacle.
then you consider that animation is CPU/GPU heavy, so will chug on anything less that tiptop hardware.
Unless there is a definite need, it really should be used sparingly