Passion for a subject is a great reason to write about it and all the reason you should need to launch a website. But if you see writing and publishing as a path to financial independence or a second income, it's important to consider how far you can go before you start the journey. New or planned publishing websites can be categorized by their potential into four loose groups, which I'm labeling: obsession, niche, commercial and the next big thing. Assigning your planned website and book to one of these four categories is tricky, and certainly doesn't guaranty that you'll fail or succeed in your goals, but should help you manage your expectations and investment.
Writing about your obsession serves an end unto itself, but if nobody shares that obsession, it's hard to see how you're going to draw readers. There may be rare, rare cases where your personal attributes or writing are just so superior to the run-of-the-mill that people would sign up in droves to read about your summer watching a snail move down your garden path. Maybe once a month somebody in the world hits it big for a day with the equivalent of snail TV out of the millions who try. If you search the web for the subject that you're writing about, and the most popular sites related to that subject don't show a dozen incoming links, it's the opposite of popular.
Niche subjects are those that draw enough interest to support a couple of titles on Amazon, but which don't draw tens of thousands of visitors a day as websites. A small niche site may draw a hundred or so visitors a day, and if the site is compelling and the subject is commercial, can help a publisher sell a couple books a day. But niche subjects are also tricky, as a topic that can draw serious traffic on a website may fail to convert into significant book sales due to the subject or reader demographics. Niche books work well when the subject is perceived as having depth, as being worthy of purchasing a book. But take a niche subject like airport parking options. It might work great as a website, drawing people from all over the country or the world checking the parking situation at their departure airport, but a very limited audience would be willing to buy a reference book on the subject.
Commercial websites for publishers are those that can draw enough visitors that they create their own business opportunity. If you have a very popular website, you can sell advertising, sell merchandise, even sell subscriptions to walled-off resources. You don't need to bring out a book to monetize your web publishing efforts, and in some cases, that book would have very little chance of selling. Any site that shows up in the top 10,000 on Alexa or Quantcast, or draws 10,000 visitors a day from Google, can usually be considered a commercial website.
If I knew how to create a website that would be the next big thing, I'd be working on that rather than sticking with this self publishing blog, which is more of an obsession than anything else. Creating a website that will draw hundreds of thousands of visitors a day has more to do with knowing people than with knowing the web. My suggestions about using the available tools to estimate the potential for a website just don't apply to the next big thing, which has no related sites until after it arrives and establishes itself. But that makes doing your homework all the more important, because if you dream is to create the next big thing, you aren't going to do it with something that a hundred other websites have already worked out.