A public IP address is set to dynamic by default. This means that an IP address might change in time. For example, if a VM that an IP address is assigned to is turned off or rebooted, there is a possibility that the IP address will change after the VM is up and running again. This can cause issues if services that are running on the VM are accessed over the public IP address, or there is a DNS record associated with the public IP address.
We create an IP reservation and set the assignment to static to avoid such a scenario and keep the IP address reserved for our services.