The reason you need a mask is because light behaves differently in water than it does in air, and your eyes focus according to how light behaves in air. That's why water makes everything appear blurry. The mask creates an air space so your eyes can focus.
This space created between your face and the mask needs to be equalised or it will squeeze to your face with increased water pressure (which increases gradually the deeper you dive) and can cause discomfort. You can find out more about equalisation in here. The reason the mask includes a nose piece is so that air you breathe in, through a snorkel or regulator, can be exhaled through your nose, thus filling you mask with air (equalising it) and reducing the squeeze.
Goggles are not suitable for diving as they only cover your eyes and do not allow for your mask to be equalised. It is a vital piece of equipment as you cannot enjoy scuba diving properly without one. Make sure you invest in a good one, which is comfortable and fits you properly.