You should consider adding a mattress pad to your bed if you live in an environment with lots of potential for dirt and spillage (i.e. with kids and pets), or if you just want to ensure your mattress stays clean. Another consideration could be if you share a bed with someone who needs a different level of firmness. For instance, if you like a softer mattress and your partner prefers a firm one, then a mattress pad could make the mattress feel soft for you without changing the overall level of support.