I've been with my current agent since 1997, and from the beginning of our relationship we talked about my career. "Where do you want to go?" was one of the first questions she asked. We talk at least a couple of times a year about my career.
Good agents know the publishing world and are aware of trends or needs long before writers. It's not unusual for editors to contact agents with whom they've worked and say, "We're looking for a book about. . . "
Agents act as buffers. That is, they have the expertise in negotiating, and authors stay out of the situation until they're ready to sign the contract. If authors have problems with the editors, the agents become their advocates. And differences do occur. It's comforting to me to know that when I have a problem with an editor I can appeal to my agent.
Editors move frequently. I read that the average editor stays 2.6 years. One editor may love your work but moves and her replacement may not and will make unreasonable demands—but you have an agent to stand up for you.
Agents work for writers;
agents make things smoother for writers.