Think about having a target market like this: If you don't have a clearly defined ideal client/target market, when you market out there in all that noise that we all hear and see every day - it's like going out in your front yard with a bullhorn and yelling out "Hey, look at me. Buy my widgets." Almost no one is going to hear you, and most likely, those who do hear you are either going to ignore your crazy yelling or aren't going to really care that you're selling widgets.
Compare that to having a target market , a clearly defined message, and then taking that message to exactly where that target market hangs out. Now you're no longer in your front yard with a bullhorn - it's more like being in the front of a room full of your ideal clients who are there specifically to listen to you and your message. They resonate with the message you're delivering and they can see how it will fix their problem or address their issues.
Think about it. When you get ready to hire someone to help you with a problem in your personal or professional life, who do you want? Don't you want to work with the expert? Don't you want to work with the industry leader?