Job searching can be a very tedious and slow process at times. By working with a recruiter you are adding another member to your team and essentially splitting the workload.
Of course, it comes with the advantage that this is what recruiters dedicate a huge percentage of their time to daily and they know the markets extremely well. With that knowledge, on top of the information you provide. They can usually find ideal vacancies quickly, that match up with your desires and skillsets.
Following from that, recruiters often have strong relationships with hiring managers and companies. This means they can influence and speed up the process at each stage as and when needed.