Identifying audience is important so you don't have to spend your time and perhaps money marketing your business to the wrong people. It is important to market the right product or content to the right people at the right time. You should target people you can help, serve, or sell to.
To determine the audience, you must determine its demographics. Features such as age, gender, location. You can also identify your audience by asking questions such as what they need, what are their problems and what solutions are they looking for etc.