A brand style guide is a granular document that includes all aspects of your brand including logos, styles, fonts, colours, icons, photography and templates. Large companies definitely require a style guide because there are large groups of people, internally and externally, who would be creating marketing materials for your organization. This is important because branding consistency is important. You can’t have products or services with your brand on it with incorrect graphics.
For Small businesses, I would still recommend a brand style guide. It is much easier to have a document on hand to give out to designers or vendors who need to know how to correctly use your logo, which pantone colours to use and which typefonts are best suited for your identity.