Sometimes it seems that anyone who owns a typewriter, word processor, or computer thinks they are an author. In spite of what each author may believe as to the quality of his or her own work, only about four or five percent of the queries will elicit a request for a submission or manuscript. Of course not. More and more, publishers are using agents to sift the wheat from the chaff. They also prefer to deal with another publishing professional rather than amateurs who don't understand the economics of publishing.
Time is money in the publishing industry.
They don't care that you have worked twelve years and poured your heart's blood into your book. They also want to know if you are a one-shot author, with no second or third or fourth book in you. Yes, an agent is a big help in getting your manuscript in the hands of publishers who might want to buy it. The really important part is when the publisher decides they like the book and they want to offer you a contract. If nothing else, the agent will advise you about the good and the bad aspects of the contract.