Using a mandala as a meditation practice is a unique and different way to meditate. It is a similar practice to visualization or using a mantra during meditation. In these practices you focus on a particular sound or sounds, or on a particular image. This is very similar to how you meditate with a mandala.
As with a visualization or a mantra, the mandalas have a certain energy associated with their image. As you meditate with that mandala you are also working with that energy. You are inviting it into your life and learning how that energy will work with you.
Do you need to mediate with a mandala? No, of course not. Then why do it? It is something different as a meditation practice. Fun and unique. The practice of holding the image of the mandala in your mind as you meditate is also a great way to exercise your visual memory and
Of course, as with any meditation practice, you meditate with no expectations of anything. You mediate just to be in stillness and silence. However if you are looking for something new and different in your practice, use a mandala and learn for yourself why they are considered to be a sacred representation of the divine.