You want to know whether you will get the disease so you can prepare yourself and your family if your test is positive. You might choose, for example, to set up financial arrangements in a certain way or change other life plans.
You want to know because the results will affect whether you marry or have children.
You think the anxiety of not knowing whether you will get the disease is worse than the certainty of knowing that you will get it.
Your health insurance will pay for all or most of the test. Or, you can afford to pay for the test yourself.