You know what "they" have always said -- if you self-publish your book, an agent won't even look at it. The theory was that publishers would think you'd already sold to the low-hanging fruit.
But that attitude is changing. Here's one author's take on the change and his pretty convincing argument that everyone needs an agent.
https://alanrinzler.com/2010/09/literary-agents-open-the-door-to-self-published-writers/