Main benefits of styled-components are listed in the Motivations page on their website.

styled-components is the result of wondering how we could enhance CSS for styling React component systems. By focusing on a single use case we managed to optimize the experience for developers as well as the output for end users.

Apart from the improved experience for developers, styled-components provides:

• Automatic critical CSS: styled-components keeps track of which components are rendered on a page and injects their styles and nothing else, fully automatically. Combined with code splitting, this means your users load the least amount of code necessary.
• No class name bugs: styled-components generates unique class names for your styles. You never have to worry about duplication, overlap or misspellings.
• Easier deletion of CSS: it can be hard to know whether a class name is used somewhere in your codebase. styled-components makes it obvious, as every bit of styling is tied to a specific component. If the component is unused (which tooling can detect) and gets deleted, all its styles get deleted with it.
• Simple dynamic styling: adapting the styling of a component based on its props or a global theme is simple and intuitive without having to manually manage dozens of classes.
• Painless maintenance: you never have to hunt across different files to find the styling affecting your component, so maintenance is a piece of cake no matter how big your codebase is.
• Automatic vendor prefixing: write your CSS to the current standard and let styled-components handle the rest.

With styled components it's possible to "enhance" CSS classes with JavaScript code because are written in JS.

const Input = styled.input`
  color: ${props => props.hasError ? 'red' : 'black'};
`;

Opposed to writing 2 separate CSS classes and control which to show with JS.

.text {
  color: black;
}

.text--danger {
  color: red;
}

<input className={`text ${hasError ? 'text--danger' : ''}`} />

On the long run it can produce more maintainable code, but it's a matter of personal opinion if it's better to use it or not.