These agents work well with beginning speakers or those who don't have a lot of leads. The professional agent has chosen to do this for a living and will choose more established speakers. They usually have a sales background and might have staffed a speaker's office before going out on their own. They are willing to mix some cold calls with current leads and have no problem following a lead to the ends of the earth. These folks usually won't touch anyone without great promo materials and video.
Many speakers make the mistake of bringing in an agent for the wrong reasons.
Just make sure your mindset can make that leverage work. According to literary agent and editorial consultant Martha Jewett, editors reject proposals that don't have a clear message or format. Original research transformed into a high-concept message. The Millionaire Next Door. The author tells the story of how the CEO and the company did what they did.