The proverb uses the case of the baby that has no other way of communicating other than crying. If the baby does not cry the mother has no way of knowing that it is in distress and this may lead to the baby actually dying. With this is mind our ancestors noted the importance of ensuring that one relays their distress to those who can assist otherwise their situation may deteriorate yet there are people who can help.
The proverb is used to compel people to air their concerns and share their problems with someone. It can also be used proactively to compel people to voice or share their desires particularly if someone else can help them attain those desires.