The major reason for vaccinating your child with a given vaccine is to protect your child from that specific disease. Thus, if your child gets the Haemophilus influenzae type B (HiB) vaccine, she is much less likely to ever get sick from HiB. Given that HiB is a terrible disease with a risk of meningitis and even death, most parents would like to prevent the illness if at all possible.
Another reason to vaccinate your child involves protecting other people who are close to your child. For example, rubella is a very mild, almost inconsequential disease for children. However, if a pregnant woman is infected with rubella during the first few months of pregnancy, there is a high risk of significant birth defects in the unborn child. So while you may not care if your child gets rubella, you certainly want to prevent your pregnant sister or neighbor from catching it from someone in your house.
Similarly, pertussis, or whooping cough, is a severe disease in infants but a milder disease in older children. Children less than six months old have not been fully immunized against pertussis and are more vulnerable. If you have a new baby in your house, you might want to make sure that your older children are vaccinated against whooping cough so that your newborn is not exposed.