Publishers are most useful in mass-market books. If, for example, you had written a new horror or sci-fi novel it would be very difficult to achieve the same level of marketing you would get from them. They also, if they're good, get your book in front of the people best suited to spread the word about it. They have marketing connections in the industry that you can't hope to match without years of work. They can not only get your book onto store shelves, they can encourage stores to make it more visible if they reallly want to sell it.
In niche markets they are less valuable. In niche markets (like adventure motorcycling, which I'm assuming yours is about) there is a very limited audience to reach, they are primarily reachable through the internet, and getting your book into thousands of brick-and-mortar stores would result in a lot of unsold copies sitting around, because most people don't care about your niche (or any other niche, by definition).
I am, of course, speaking in large generalizations... The short version is, they save you the trouble of marketing it yourself. Do you really want to spend months marketing and selling your book in order to make it sell well enough to even have a hope of real income off of it? Or, would you rather get to work on the next book (or simply living) and let a professional work on that for you in exchange for a cut of the profits?
With that said, there are better ways to get your book printed than Amazon that will still get your book ON Amazon (both printed and kindle versions) however, addressing that issue is far beyond the scope of this question.