I'm sure you know what a consultant is. The dictionary defines a consultant as "an expert in a particular field
who works as an adviser either to a company or to another individual." Why would a normal business need business consultants?
Business consultants are more than just experts, they're specified in knowledge in a certain path. Where others may dabble in that area, consultants usually have a bright knowledge in the path they've chosen in business.
Let's get an example. If you look to contract a SEO consultant, it's because you know that the specific person has a lot more knowledge in SEO than a product marketer. A SEO consultant can take product marketing, and twist it up fancily to attract web users. Online, a SEO consultant would work well for your product marketing, whereas a product marketer would work best offline. Get the picture?