A niche is a speciality. Your speciality is the thing you will be known for, hopefully even famous for.
It’s important to be specific in your niche so that you can become the foremost photographer in that specialty. If you try to have too many specialties people won’t remember what you do. For example, if one photographer specialises in weddings, and children, and real estate photography, and another photographer specialises in spiders. People will remember the spider guy, and the other photographer will likely blend in with all the rest.
This might seem like a terrible oversimplification to think of the typical customer as unable to remember who does what type of photography. But clients are going to type cast you, and only think of one photographer as great for real estate, and another one who is great with children, and yet another one as the go-to photographer for spider photos.
If you really want to have multiple specialities, it would be a good idea to have different websites and marketing for each one. An example would be a photographer whose speciality is shooting food. She can easily sell her work to food magazines and websites. But let’s say the same photographer also does very artistic automobile photography which she sells to stock sites and industry publications. She will not likely sell the food magazines any automobile photos, or vice versa.