You must build a niche website if:
Understand this: You can master all of these subjects, but if you don't build a niche website you may never be successful.
For instance, say you have a website on dogs. You have many pages about the subject of dogs. This is a very wide niche for one person. Someone else has a site about poodles. They have dozens of articles covering only poodles. This is a tight niche.
Now, when someone searches for poodles guess which site is going to be listed higher in that search engine's results pages? You got it the poodle site. The poodle site is much more focused on poodles. Its niche is poodles.
You could, if you had the time, create dozens of pages for each type of dog. When someone searched for almost any dog you could expect your site to be listed somewhere high in the search engine's results pages. But there is only so much one person can do.
Big companies can afford to create many articles on many subjects and therefore be seen in many search engines results pages. They usually don't do this, but they could if they wanted to. Big companies seem to prefer to advertise.
Us "little guys" don't have the marketing budget that a big company has. So we choose topics that are not too large for the amount of time we are willing to put into it.
How large is too large for a niche? Say you loved Philadelphia and you wanted to build a site about it. Would you say that is too large? Sure it is! Ok, let's take a smaller niche. Say you love Philly because of the bar scene there. Would a site about the Philly bar scene be too large? Probably it depends on how much time you had to spend on it. Ok, let's take a smaller niche. Say you really like Philly because of the brewpubs. Would a site about Philly brewpubs be too large? Maybe it depends on how much time you had to spend on it.
So how do we (little guys) decide upon a niche for a website? The short answer is, "start small." You can always create another site. In the above example you might want to start with creating a very small niche site about one brewpub In Philly, say... the City Tavern.
The City Tavern is a very interesting and historic tavern. George Washington and Thomas Jefferson used to hang out together there around the time of the signing of the Declaration of Independence. Fascinating stories!
If you did choose the niche of the "City Tavern" and you ran out of topics to write about, you could possibly start writing articles on other related topics and still be in the same niche. Get it?
If you are building a website for a small business or a hobby and/or if you are the only one working on a site be sure to build a niche website. Choose a niche you know. Choose niche you are passionate about. Choose a niche that is SMALL. You will get more visitors and more targeted visitors from the search engines when you build a niche website. See How to Build a Niche Website for more info.
Keep your website focused on a tight niche. The tighter your focus the better.
You can always create another related site and link the two together.