I find the world of marketing and branding agencies very confusing. You ask them what they do, and you get a description of a process that sounds and looks very similar to everyone else you ask the same question. But in practice, people are actually very specialized. Defining the personality of a brand, creating the competitive strategic positioning of a company, making the pitch deck, generating leads, designing ads, running online campaigns, designing logos, etc .etc.
The best strategy to find out what people do is to ask them to describe a project, and see where in this whole jungle they played a role, and most importantly, at what stage in this description see you light up the eyes of the person you are considering working with.