You want to explore as many options as possible, get as many offers as you can and select the best position regardless of the source.
Employment agencies have a large abundance of potential career opportunities.
They work with a myriad of companies from large to small in all industries. They have long standing relationships with companies that make them able to generate interviews for candidates based on trust from the employer. The employer knows that if the employment agency sends a candidate, that candidate has met the screening criteria laid out by the company. This means that the chances of the candidate being the right fit is much greater than those who apply from other sources. Many of these positions never even receive public exposure - the company goes directly to the agency to fill the opening.