Styles help you apply formatting — such as colored and shaded blocks of text, and customized headings and lists.
You can't really argue with the power of formatting in a document.
If all text looked exactly the same, reading it would be a struggle. You'd lack the signposts you depend on, such as bold headings that announce main sections, numbered lists that tell a sequence, and special shading and fonts for areas you want to call out.
Word styles exist to help you do both simple and complex formatting, and do it with minimal fuss. See how styles differ from basic formatting, and how to apply them.