By listing all of your sources of income against all of your monthly expenditures (from required expenses like mortgage or rent payments to discretionary spending like eating out or going to the movies), you get a true picture of your personal cash flow, which will allow you to make better and more informed financial decisions. An accurate budget will also help you to answer that ever elusive question, "Can I afford it?"
You might choose to break down your expenses even further, between things you need to have and things you want to have. For instance, if you drive to work every day, gasoline probably counts as a need. A monthly music subscription, however, may count as a want. This difference becomes important when it’s time to make adjustments.