A specialist has expertise specifically in your field. They are talking with Hiring Managers and Candidates within your industry every day, so they are very knowledgeable of market/industry trends; upcoming sales, acquisitions, and mergers; key players; company and candidate reputations; confidential job openings. It would behoove you to work with a recruiter who truly understands and knows your field, from both a hiring and a job placement perspective.
Company-- It is your prerogative to work with various recruiters; however, you really only need one good, honest recruiter...the longer you work with your recruiter, the more insight they will have on your Company and the better they can recruit for you. Sometimes multiple recruiters working on the same position can harm your reputation - your Company can be perceived as "desperate" to fill a position; questions arise about why your Company cannot attract someone to work for it; candidates can become agitated by hearing about your same opportunity over and over again.
Candidate-- If you are confidentially looking for opportunities outside of your Organization, it would be wise to work with only one good recruiter you trust. Even if you are openly looking for new opportunities, you do not want your name floating around every company in the market. If you're presented to the same Company by multiple recruiters, you may be perceived as "desperate" for a job and questions arise as to why no one has hired you yet.
No. The Hiring Company is responsible for fees.