These are not always necessary, but they can certainly be helpful! If there are factors in your medical or family history that you think might make getting pregnant more difficult or complicate a pregnancy, then your doctor can discuss these with you and give you advice and help to optimize your health before becoming pregnant.
If you have chronic medical problems like diabetes, hypertension, or other conditions that require you to take a medication, a preconception visit is a great idea. Getting medical conditions optimally managed with medications safe to use during pregnancy is the goal.