That’s a good question John!  In my experience it most certainly does.  When you join a reputable agency they undertake a vetting process before you go to meet any of their clients.  This includes checking your documents, interviewing you and verifying your references.  You also have the option of updating your DBS and obtaining a medical check up etc.  By doing this process, the agency is then able to quickly put you forward for subsequent jobs as they arise.  Likewise the client (who in our world is often on a quick timeline) doesn’t want to wade through hundreds of CVs or vet the candidate, instead they want to be presented with a few selected CVs, meet the candidates, where appropriate trial and then hire.  This process is expedited through using an agency, both both employer and employee.
Of course, not all candidates are successful with applications for work, and its important to realise that just because you have joined an agency it doesn’t mean you can sit back and wait for the job offers to role in – you still need to be pro-active and responsive, updating your CV and making good relationships.  A good agent should be treated with respect as they are the gateway to connections with new employers, so make sure you treat them well, as like an elephant – we never forget! 🙂