What if component A and component B need that button? You will comp-a,comp-b{.button{}}? Will you save in comp-a-and-comp-b.scss? What if you need in comp-c and comp-d? As you can see your button is not tight to that comp.

I prefer to have a button class, for example in react that when instanced renders all the time the same. It boundles it's css within, just like this lib.