A Stomach Sleeper is a person whom sleeps on their belly for the majority of the night.
They tend to need a firm sleep surface so that their back is supported in a safe and non-painful manner.
A soft sleep surface would cause an unnatural bend in your back causing pain and loss of sleep.
Pillows are not necessary for most stomach sleepers, however, if you prefer having a pillow use a very low loft firm pillow so your neck alignment is not affected.