To be great, your contribution needs to be identifiable. You need to rule the market that you are in. In order to do that, you definitely need to niche down. This is especially so when you are starting out and you want to make a headstart. It is most of the time the case for most to want to target every person that they can possibly could, only to be disappointed with the number of whom they managed to reach.
Here is the thing, people, especially consumers are pretty straightforward and efficient. They like things to be simple and easily understood. So for example, if they need help with taking of their wedding photos, given a choice of a photographer who shoots for weddings and someone who shoots everything, they are more likely to go with the former. The latter could be someone who is very experienced but the former is more likely to get the job because of the perceived expertise that he/she might have.
The earlier you niche down and know for certain who your customers are, the more will you be rewarded.