Is this test really necessary? What is it testing exactly?

My doctor wanted to give me the glucose test and I was wondering what it's supposed to test for exactly. I know it's not painful, but is it really necessary?

    The three hour glucose test is completely necessary, but a little inconvenient. What it does is it lets you/the doctors know exactly how your body is processing it's glucose and at what rate. It helps determine if you are at risk or have gestational diabetes.
    About Ngondo Mulumba
    Born: Edison, New Jersey
    Current: Edison, New Jersey
    Birth: December 31
    On since: Apr 27, 2014