I personally know many professional writers who do not use a literary agent. It is true that an acquiring editor will probably be quicker to read a manuscript represented by an agent whose judgment she trusts. But ultimately, what sells a book is the book itself. Editors frequently disagree with agents' assessments of manuscripts. If an editor doesn't love your book and think she can publish it successfully, she won't buy it, no matter what your agent says.
Agents make their living by representing successful writers.
Unless they're just starting out, they're probably not very hungry for new, unpublished authors. If I were starting out now, I'd concentrate my energy on getting a publisher first, and not worry about getting an agent until after I had a firm offer from a publisher. It's impossible to predict how much money a single book will make. But for most of us, this kind of money is a lovely fantasy. In short, you should be warned, if you don't know it already, that most of us published writers do NOT earn a grand living.