When it comes to most US recruiting agencies there are several processes you should be aware of as someone dealing with recruiters, how you're contacted and how they approach that connection will help you understand what they are doing and have to offer.
Lead Generation Recruiters
The very first step in recruiting is generating leads, that is getting as many qualified people into their system to offer prospective employers decent potential hires to pick from.
This is the unpleasant part of recruiting. Cold calls, dragnet recruiting, recruiters by the dozen at every event in their respective markets, etc. Their job isn't to fit you into a job, or to get you a job. Their job is to get you into their system to hopefully land you a job later. (ie. they aren't trying to hire you for a specific role, they are trying to actually build up the pool of people to pull from to potentially hire.)
It's a mutually unpleasant job for both sides. In high demand fields you get nearly harassed by the sheer volume of these people contacting you. As the recruiter you get the a bunch of people demanding you stop calling you or just cursing you out, etc.
These are one of the most important people in recruiting, and as a potential hire the person you want on your team more than anyone else. Account Managers are the people who actually work with companies negotiate contracts, and take what jobs are being hired for to the placement recruiters. If you get in good with an account manager you could potentially get first preference for new positions.
This is the recruiter who actually vets you to make sure you're not lying through your teeth. They are also the recruiter who works with you personally to find the jobs the account managers are working and get your resume to them. Typically this will be who you ultimately work with to get your job.
You can tell a good placement recruiter from a bad one very quickly. A good recruiter will want to get to know you, preferably in person over 30 minutes to an hour. That means they vet people, which means they also likely vet the potential employers. Just listen, if they don't feel like a good fit, find someone else.
What you're seeing
Cold calls, canned emails, etc. are what many call "drag net recruiting" basically it's a lead generation recruitment method where they just shotgun an email template to everyone that has a certain skill, word in their profile, job title, etc.
When not looking, it's best to just ignore these. Find the recruiters that are active in your area visiting user groups and have placed peers in good jobs. Get their names and network. Have your name in their systems. They'll probably check in once per 3 to 6 months to see if you're looking, update your info in their system, etc.
When the time comes to look, give them a call and get the ball rolling. The account manager is paid to get employees hired by companies, the placement recruiter is paid to get their clients jobs. Both sides have rules that they don't get their full pay for placement until you hold the job for a period of time. So they want you to be happy, otherwise they could lose both you, and the company as clients, and their pay.