A niche is a group within a group. It narrows down broad categories. This helps you in many ways including greater chances of being found in search results.
For instance, how could you market Cars? You need to narrow that down. Foreign or Domestic is ok but you could also say 2 door cars. How about 2 door domestic sports cars?
How about 2 door domestic sports cars.
Well, if you are talking about marketing it is to make money either directly or indirectly. This means that your niche should be chosen carefully.
There should be enough of a demand of the niche you pick, that people are searching for it in numbers which will give you a decent flow of traffic when you rank on the first page on search results. You need to do a little research in order to figure out what terms people are using to search your niche. Niche keywords can be found in search terms sometimes typed like a sentence or question.
Those same keywords are used in multiple ways by different people. These words are key to the many terms that people put in the search engine. Their popularity makes them keywords for that term. What keywords are related to the niche you have in mind?
Marketing a niche is better for many reasons