The world of the internet seems to have changed the way business works, for better or worse. In the old days customers were only obtained through retail shops and potentially mail order catalogs. This was very time consuming and made exploring for new companies quite difficult. However, now thanks to search engines like Google and Yahoo, people can search for product categories and find a huge list of businesses that can provide them to customers. This is a great way to generate sales and bring in new customers, which is why one reseller SEO guy has become quite popular for helping businesses to get attention on search engines.
The bottom line is that every search engine has an algorithm that determines which results are the best for a certain phrase or keyword. Most of these algorithms have not been publically released, so it takes a lot of time and effort to figure out exactly how the ranking system works. Any company trying to do this will likely waste huge amounts of time that could be invested elsewhere, rather than simply hiring an outside consultant. Of course any time a consultant is hired, it takes a lot of trust in the company to do things right.
Giving them control over details and features of the website can easily mean potential problems, so it is wise to only make moves that seem logical and trustworthy. Of course if a company is not an expert in this subject, then it can be truly difficult to determine whether something is working or not. In this case then the best bet is to simply find a company with a reputation for doing things right. They obviously will not want to risk ruining this reputation, so will do their best to get consistent and positive results.