Generally, the concept behind niche marketing is that it's a whole lot easier to stick a pin into a particular spot rather than throwing a blanket out and hope you hit something. Some refer to it as VAs who try to be all to everyone and end up being nothing to anyone.
My entrepreneurship class described it as a kind of arrow marketing. You have a wall between you and the rest of the world of potential clients out there. If you poke a hole through the wall, a very specific, very tiny hole, you have a pretty good chance of poking through the wall and out the other side. Once on the other side, that hole starts to expand and get bigger and you stand a very good chance of landing potential clients. However, if you choose a big huge section of wall and ram into it with a huge section of what you think will get you through the wall, chances are you are neither going to pierce the wall nor land any clients. Too big a section to try to penetrate.
The nice thing about niche marketing is you're not limited to one tiny, little, specific hole. You can pick a whole bunch of tiny little holes to market to and chances are you are going to land clients. It also enables you to pick target markets that you
want to work for, another handy benefit.
One doesn't have to have a niche to market to and that may work for you. One thing I have noticed is a lot of VAs kind of 'discover' their niche as they go along and it's always nice to see when that works out in their favor!