The most common misunderstanding we run into is that a logo and a brand identity are the same thing. They aren't. A logo is one visual mark. A brand identity is the system that mark lives inside, the type, color, layout rules and voice that make everything you produce feel like the same company.
A useful identity system gives your team answers, not just files. When someone needs to design a proposal, a sign, or a social post, they should be able to open the guidelines and know what to do without asking.
At minimum, we consider an identity system to include: a primary logo, secondary marks, a type system, a color system with usage rules, brand voice guidance, and templates for the touchpoints you actually use most.
If you're evaluating a branding proposal that includes 'a logo and some variations' but no system, ask what happens the first time someone on your team needs to design something that isn't already in the file.