ShadowDOM removes the need for a component user and a component author to collaborate in order for styles to be encapsulated. This, in turn, enables easier distribution and usage of generalized components.
Exactly, how many times do you want to use a component but find it near impossible because it's written with a framework, or uses CSS styles that conflict with your own, or is written for use with a different version of the very framework you're using... this is why we keep re-inventing the same dumb web components over and over again.