The seven bowls are full of the wrath of God. They are each in its own way unspeakably horrible, though the hail may be the worst.
Three times this chapter says something like, "They blasphemed God because of the plagues, but they did not repent." People who receive the wrath of God receive it because they would rather receive it than to acknowledge God as God (i.e., repent)! Wrath does not fall upon innocent people. Even the fierce wrath of God does not bring about humility and repentance.
Sprinkled throughout this futuristic terror are reminders to be ready, to watch, to stay dressed. Don't pretend God's wrath will never happen. Be prepared.