Of course, now you must differentiate between which expenses are "needs" and which are "wants." Basically, any payment that you can forgo with only minor inconveniences such as your cable bill or back_to_school clothing is a want. Any payment that would severely impact your quality of life, such as electricity and prescription medicines, is a need.
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?"