An agency offers a lot of added value if you want to outsource advertising on bol.com to an expert. For example, they can arrange the entire campaign management in sponsored products for you as well as giving you advice on your content or marketplace strategy. See this link for an overview of the agencies with which bol.com has a partnership.
As you can see above, there are many different agencies that bol.com works with. There are a number of agencies that have received special certification. They are recommended by bol.com because they have large-scale expertise in advertising via bol.com. They help you set up a strategy and give you advice on how to optimise your visibility.
These agencies offer you the option of fully outsourcing advertising via bol.com. In short, they take the work off your hands. You can recognise these agencies by the special Advertising Certified label they use. See below which agencies have this label: