Lets Understand first, What is Style-Guides ?
A style guide is a document gathering all elements of your brand’s visual style. As Susan Robertson from the popular design blog A List Apart notes, a style guide is a “one-stop place for the entire team—from product owners and producers to designers and developers to reference when discussing site changes and iterations.”
It helps us to maintain
Consistency - a unified visual experience is one of the main qualities of brand identity that help you create trustful relationship with your audience.
Shared Vocabulary - When all your team member have one document they can refer to, the work really becomes collaborative.
Onboarding - New Designer can be involved in making design decisions from the very first day & don't have to constantly bug older employees.
Code Standardization - UI style guides help create consistent HTML,CSS & JavaScript code so front end developers can follow the same standards designer do .
Conclusion
Every project is usually different but I believe all of them can benefit from the style guide if it is defined with care and with a design system in mind.