If you are delivering at a hospital, it will be recommended that you have one.
When you go to the hospital and are admitted for labor, one of the first things your nurse will probably do is place an intravenous line, or IV.
An IV is used to give you fluids and medications directly into your bloodstream. This can be helpful if you are not able to drink and stay hydrated or if you need certain medications in labor such as Pitocin, which can be used to make your contractions stronger and also to help treat bleeding after you deliver. Some women request that an IV not be placed, and while this is usually against hospital policy, most will acquiesce after you are counseled about the risks. Risks of not having an IV in place include not being able to give you emergency medications when needed or delaying an emergency C-section (because time would be lost placing the IV then).
Blood transfusions are also done using an IV, and if needed in an emergency situation this would also be delayed if you did not have an IV.