Many recruitment agencies advertise jobs on their websites, where they will be able to reach a wide audience by putting posts and adverts out into the general job market. If you want to apply for one of these roles, it is a good idea to know the recruiter first, so you will be making an application that is known. The recruitment consultant will then recognise you, may give you preference, and it will most definitely work in your favour to have an ongoing relationship with that consultant.
It totally depends on the job market which recruitment agencies you will use – some will be more relevant to your job search than others. If there are agencies operating in your field, see if you can set up meetings with them. Meeting a recruiter face to face is beneficial because they will be happy to represent you, and will have more confidence in your applications.