An SEO Agency is one that focuses on providing proper keywords and content for a business to be found easily on the Internet Search Engines Results Pages. SEO requires good research of what a company does, what they sell or promote and where they want to go for the future.
Research of all of those topics will help your SEO company determine what's important to you and your company. Once that is determined, the SEO company will then start to find the most dominant "keywords" or "keyword phrases" for your particular situation. An example would be a company that sells shoes only - not boots or sneakers and has no intention of selling them. A good SEO Agency will research all the various keywords that can be associated with just Shoes.
Once they've determined the best keywords that apply to "shoes", then a content team will work on creating excellent content for that shoe company. The goal of an SEO Agency is to get the company's website found easily with "Organic" search results (Not Pay Per Clicked Ads). That means their content is so good and focused that all the search engines rank the website well and it moves into page 1 of search results without having to pay for that.
If an SEO Agency wants to also include pages and topics about boots, sneakers, etc., step away! They didn't listen to what your company is all about.
The good Agency will write content and gear their keywords around shoes only and devote pages specifically to various types of shoes. The Keywords or Keyword Phrases will then be the URL Pages, Titles, Descriptions and incorporated within the content. This systematic approach to specific keywords about Shoes will help that SEO Agency get excellent results for the business owner.