You will notice that these are large enough groups to be markets but are specific enough to have some very similar behavior patterns. That’s important. Remember, this is a market to which you want to sell your product. If you want to sell a lot of your product, it makes sense to pick a market that has enough people to eventually support your growing business. The specific part is important because you want to be able to easily identify those people for your research.
Essentially, "why should this thing exist?"