Styles are preset ways of formatting text and defining structure in a document.
Styles are much quicker at changing the appearance of text, you don’t need to change each item of formatting individually (e.g. font, size, colour and emphasis).
Styles give your work a consistent appearance which makes it look neater and helps the user understand the importance of sections of your work.
Styles can be updated and any occurance of the style will change automatically, which means large documents can be quickly changed to match new formatting standards.
Styles can be used to automatically generate contents sections in larger documents.
Applying Styles in Google Docs
Applying Styles in Microsoft Word