Manufacturers are associated with products, providing web site, contact, and descriptive information. Each product that is created can be associated with a Manufacturer.
For stores where manufacturer information is not necessarily relevant, you do not neeed to create them.
It is not necessary to create manufacturers prior to creating your catalog. Products can be re-associated to manufacturers after all the product records have been created. However, creating the manufacturers first can save a lot of extra work if you already know that you will be using this information.